How to Install a ‘65 Mustang Wiring Harness

Electrical work is considered one of the most difficult parts of restoring a classic Mustang. And while replacing a complete wiring harness requires time and patience, the payoff is worth it. After all, nobody likes chasing ghosts in the electrical system.

Modern wiring is better built and insulated to provide a consistent electrical current. Higher quality materials result in longer-lasting wires. Most new wiring harnesses also include extra wiring and connections for modern accessories and upgrades. If you’re going to take the time to install a complete chassis harness, it’s a job you only want to do once.

In this guide, we’ll walk you through the preparation, placement, and connection of each harness with easy-to-follow video, steps, and images. If you want to skip ahead to a certain part of the vehicle, you can use the links below.

Preparation | Gauge Cluster | Engine Bay | Trunk | Final Connections

Before You Start

Before you settle on a new wiring harness, take some time to decide whether or not you’ll be upgrading other electrical components. Taking on installation projects like an electric fuel pump or new sound equipment might require extra circuits. Installing fog lights on your classic Mustang could require additional wiring as well.

We also recommend picking up wiring diagrams for your Mustang's model year. Their larger print size, detailed schematics, and correct color-coding will reduce confusion while you navigate your rewiring project.

Wiring Harness Installation

Follow these steps along with your harness’ manual and wiring diagrams. We rewired Project Betty, our 1965 Mustang, using Painless Performance’s 22-circuit complete wiring harness. The whole process is similar for 1966 Mustangs, though.

It’s also worth mentioning that our car is gutted while we replace the wires in our video. This means you’ll have to rely on the manual to connect the harness to items like the engine and transmission. If you’re installing the Painless harness and no longer have the manual, you can print a PDF copy of it here.

Required Tools:

Preparation

This section focuses on separating the new harness and removing the old wires. These steps will help you come up with a plan to stay organized during the install.

Step 1: Read the Manual

Before you get started, take the time to read the manufacturer’s manual. This will give you an overview of what the installation of each harness section requires. Knowing what to expect will help you better plan your project.

Step 2: Organize Your Pieces

Take your harness out of the box and unwrap it to get a feel for where to find the various connectors and wires. Leave the pigtails, connectors, and terminals in their bags until you’re ready for them.

Step 3: Remove Battery

Disconnect and remove the battery to give yourself as much safe space to work as possible.

Step 4: Start to Remove Factory Wiring

Starting from the headlights and moving back, begin removing the factory wires and harnesses from your Mustang.

Step 5: Cut Ground Wires But Leave Terminals (During Entire Removal)

When you run into ground wires, cut them from the factory harness but leave the terminals bolted in place. This will mark the factory ground locations and keep the OEM hardware from getting lost.

Step 6: Remove Ignition Harness Nut

Don’t forget to remove the ignition harness nut securing the wiring in place behind the gauge cluster.

Step 7: Remove Trunk Harness

Release the tabs holding the trunk wires in place with a flathead screwdriver. This will allow you to remove the harness with ease.

Gauge Cluster Harness

Rewiring your gauge cluster is generally a straightforward process. If your harness is compatible with multiple Mustang years, make sure you use the wires intended for your specific model. If you have installed a classic Mustang rally pac, make sure to connect that wiring as well.

Step 1: Start with Fuel Gauge

Slide the ring terminals for the fuel gauge wires onto the fuel gauge bolts and secure them in place with nuts.

Step 2: Install Side Panel Lamps

Insert the panel lamp bulbs into the designated sockets on the harness. Then, press the panel lamp assemblies into the holes with blue inserts on the fuel-gauge side of the cluster.

Step 3: Connect Oil and Alternator Gauges

Insert bulbs into the oil and alternator gauge sockets on the harness.

Press the oil bulb assembly into the top hole on the fuel-gauge side of the cluster. Do the same on the temperature-gauge side with the alternator bulb assembly.

Step 4: Wire Opposite Side Panel Lamps

Insert the remaining panel lamp bulbs into their sockets on the harness. Press the panel lamp assemblies into the two holes on the temperature-gauge side of the cluster.

Step 5: Wire the Turn Signal Indicator

Insert a bulb into the turn signal indicator harness socket and press it into the remaining hole beneath the alternator gauge.

Step 6: Wire the High Beam Indicator

Insert a bulb into the high beam indicator harness socket. Press this assembly into the remaining hole beneath the oil gauge.

Step 7: Attach Constant Voltage Regulator

Attach the male and female plugs to their respective connectors on the constant voltage regulator.

Step 8: Wire the Temperature Gauge

Slip the ring terminals for the turn signal wires onto the bolts next to the turn signal indicator. Secure both terminals in place with a nut.

Underhood Harness

This harness will connect to items like the voltage regulator, headlights, washer hose, and horn. Be prepared to strip and cap many wires with a variety of different terminals and connectors.

Step 1: Take Capacitor From Factory Harness

Locate the plug for the voltage regulator on the factory harness. Pull the attached capacitor off the factory harness without damaging the capacitor or wire.

Step 2: Attach Factory Capacitor to New Harness

Strip the tip of the yellow wire jutting from the voltage regulator on the new harness.

Clamp one of the supplied female connectors onto this wire and connect it to the factory capacitor’s male connector.

Step 3: Add a Ring Terminal to Voltage Regulator Ground

Locate the black wire on the regulator, insert the exposed wiring into a ring terminal, and clamp it down.

Note: If the regulator plug’s black wire is insulated to the tip, you’ll have to strip some off to attach the terminal.

Step 4: Prep Headlight Plugs

Locate the gray ground wires tied to both of the headlight plugs. Strip the tip of both grounds and add a female butt connector to each of them with a pair of pliers.

Step 5: Position Underhood Harness in Place

Starting with the dash wire cluster at the driver side of the firewall, wrap the harness around the engine bay. It should run to the voltage regulator and driver-side headlight assembly. The harness then crosses to the horn and passenger-side headlight and finishes up at the alternator on the passenger side of the firewall.

Step 6: Set Driver-Side Headlight Harness in Place

Thread the driver-side headlight and parking light harness through the hole at the front of the engine bay.

Step 7: Connect Harness to Voltage Regulator

Plug the voltage regulator into the voltage regulator connector on the wiring harness.

Step 8: Mount Regulator and Attach Ground Wire

Mount the regulator in its original position. Attach the regulator ground wire to the top bolt using the ring terminal you added in Step 3.

Step 9: Attach Radio Suppressor

Attach the radio suppressor’s mounting ring to the voltage regulator’s bottom bolt.

Step 10: Plug In Washer Hose Wire

Strip the tip of the washer hose wire and add a female spade connector. Plug the connector into the original washer hose assembly.

Step 11: Connect Horn Wires

Slip the yellow horn wire through the hole at the front of the engine bay and plug it into the driver’s side horn. Repeat this step on the passenger’s side for the second horn.

Step 12: Attach Headlight Grounds

Attach the driver-side headlight ground to the voltage regulator’s top bolt. This is the same bolt we attached the regulator ground wire to in Step 8.

Attach the passenger-side headlight ground to the factory ground location near the hole for the horn wire (pictured).

Step 13: Set Passenger-Side Headlight Harness in Place

Thread the passenger-side headlight and parking light harness through the hole at the front of the engine bay.

Step 14: Tuck Fan Wire Out of the Way

There should be only one gray wire that runs across the front of the engine bay remaining. This wire is for an aftermarket electric fan should you choose to install one. If you aren’t installing an electric fan, tuck or tie this wire safely out of the way.

Step 15: Connect Parking and Headlight Harnesses

Locate the passenger-side parking light harness under the front fender. Feed the harness through the small hole in the headlight housing (it’s hard to see). Plug the corresponding wire connectors into each other and push the wires back behind the housing. Repeat this step on the driver’s side.

Step 16: Connect Wires to Alternator

At this stage, you can connect the harness to your alternator. If your engine bay is empty like ours was, set the wires out of the way until you’re ready to connect them.

Step 17: Widen Firewall Holes for New Harness

Widen the existing holes on the driver side of the firewall to 1¼” using a hole saw and grinder. This will allow the new harness, connectors, grommets, and plates to install in place.

Step 18: Feed Harnesses Through Grommet and Plate

Push both wiring harness connectors through the supplied grommet and plate. Feed the harness connectors through the firewall closest to the side of the engine bay and push the grommet into the hole. Secure the plate in place with the supplied hardware.

Pro Tip: If you’re having trouble slipping the connectors through the grommet, a small amount of lubrication will make your life much easier.

Step 19: Attach Fuse Block Mount

The new fuse block will mount behind the gauge cluster on the sheet metal to the left. With your kick panel in place, position the fuse block mount just above the panel. Secure the mount in place with the included self-tapping screws.

Main Harness

Connecting the main harness to the components in the dash and doors will be the most physically demanding part of this installation. Be prepared for tight fits, squeezes, and maneuvering tools into difficult-to-reach places.

Step 1: Position Main Harness in Place

Feed the main harness up under the dash and out through the gauge cluster hole. Continue to pull until the connectors are situated roughly where they’ll need to be.

Step 2: Remove Wiper Switch

Remove the set screw from the bottom of the wiper switch knob, and pull the knob out of the switch. Then, remove the ring located behind the knob and pull the switch out of the dash.

Step 3: Connect Wiper Switch to Wiper Harness

Locate the individual bag containing the wiper harness. Attach the five wiper harness connectors to the back of the switch.

After these five wires are installed, you should have two connectors left. These are for the ground wire and positive terminal. These wires are found on the headlight section of the main wiring harness.

Reference your installation manual here to ensure you make the correct wire-to-switch connections.

Step 4: Reinstall Wiper Switch

Thread the wiper switch back into its hole and secure it in place with the ring, knob, and set screw.

Pro Tip: Install the headlight switch first to make it easier to install the wiper switch.

Step 5: Attach Main Harness Grounds

Locate the two main harness ground wires and the wiper harness ground wire. Then, remove the factory ring terminal that was cut from the ground wire in Step 5 of the Preparation section. Sand the ground hole and the area around it to remove paint or corrosion.

Line up all three ground wire ring terminals with the ground hole and bolt them in place.

Step 6: Attach Wiper Harness to Factory Connectors

Thread the end of the wiper harness out through the bottom of the dash. Locate the factory wiper harness connector and connect like wires to like wires (white to white, red to red, etc.).

Step 7: Connect Remaining Wiper Harness Pigtails

Connect the final wiper harness pigtails to the blade terminals on the wiper motor.

Step 8: Pull Harness Through Dash

Pull the rest of the wiring harness behind the dash to make the rest of your connections.

Step 9: Connect Ignition Switch Wiring

Attach the gray ignition wire’s ring terminal to the stud in the middle of the ignition plug. Then, connect the ignition plug to the back of the factory ignition switch.

Step 10: Connect Harness to Turn Signal Wiring Switch

Position the turn signal wiring and switch over the brake release handle. Plug the white connectors on the main harness into the bottom of the turn signal switch.

Step 11: Mount Fuse Box

Press the fuse box into the fuse box mount you attached in Step 19 of the Underhood Harness section. Pull the harness through the dash to keep the wires close to where they will connect.

Step 12: Set Driver Door Jamb Switch Wires in Place

Push the wires for the door jamb switch through the hole in the door jamb. You can leave them here for now or connect them to the switch. Jump to Step 22 to see how to connect the door jamb switch. Or, you can wait to connect both switches at the same time.

Step 13: Connect Main Harness to Brake Light Switch

Attach the brake light wires’ red connectors to the brake light switch.

Step 14: Connect Main Harness to Underhood Harness

Attach the white connectors on the main wiring harness to the engine harness connectors. They’re the same ones you fed through the firewall in Step 18 of the Underhood Harness section.

Step 15: Connect Foot-Operated High-Beam Switch

Attach the black switch on the harness to the foot-operated high-beam switch.

Step 16: Reattach Flasher

Reattach the flasher into the factory location behind the vent.

Step 17: Attach Cigarette Lighter Wire

Strip the tip of the yellow cigarette lighter wire and add a ring terminal. Slide the terminal onto the stud at the back of the lighter assembly and secure it in place with a nut.

Step 18: Feed Harness to Engine Bay and Secure It

Feed the main harness’ engine-bay-end into the second hole cut in the firewall. Then, head to the engine bay and slip a plate onto the harness followed by a grommet. Push the grommet into the plate and the hole in the firewall.

Step 19: Position Radio Wires

Return to the interior and find the red and yellow radio wires on the main harness. Thread both wires up and through the hole where the radio sits to keep them ready for the radio install.

Step 20: Connect Heater Motor Wires

Strip the tip of the yellow, black and gray, and brown wires for the heater motor and add a female spade terminal to each wire. Connect the wires to the blower switch: yellow connects all the way to the left, black and gray to the middle, and brown to the right.

You could also wait to connect these terminals if you plan to replace your heater box and controls.

Step 21: Attach Courtesy Lamp Wire

Position the courtesy lamp under the dash and connect it to the harness with the bullet connector.

Step 22: Connect Passenger Door Jamb Switch

Feed the two door-jamb wires on the main harness through the hole on the door jamb. Plug them into the door jamb button and secure the button back into the door jamb hole. Connect the door jamb switch on the driver’s side if you didn’t in Step 12.

Rear Harness

The rear harness will be a welcome break from rewiring the interior. The overall process is simple and shouldn’t take very much time.

Step 1: Feed Main Harness to Trunk Area

Slip the end of the main harness through the hole on the driver's side of the trunk.

Pull the harness through the trunk hole until the connectors and wires are in position for installation.

Remove the driver-side door sill plate. Secure the harness in the recessed area under the door sill or under the carpeting in the cabin.

Step 2: Connect Taillight Wires

Locate the factory taillight pigtails on the driver’s side. Strip the tip off the green and black taillight wires on the main harness.

Connect the green wire on the harness to the green wire on the pigtail with a butt connector. Then, connect the black wire on the harness to the black wire on the pigtail with a butt connector.

Plug the taillights in and repeat this step on the passenger’s side.

Step 3: Connect Backup Light

Press the driver-side backup light’s power wire connector into the corresponding pigtail on the harness. Repeat this step on the passenger’s side.

Step 4: Plug In License Plate Light

Plug the license plate light connector into the main wiring harness.

Final Engine Bay Connections

While the engine bay would normally still be assembled, we chose to gut our Mustang to provide a better view of the wire placement. That said, we still go over the engine and transmission wires, but you'll need to follow your harness' manual to connect them.

Step 1: Connect Harness to Main Fuse

Strip the ends of the two gray wires designated for the battery mount and add a ring terminal to each wire. Mount the fuse to the engine bay and secure both gray wires' ring terminals to the fuse bolts.

Step 2: Connect Black Battery Cable Wire

Attach the black wire’s ring terminal to the battery cable side of the solenoid.

Step 3: Connect Red Battery Cable Wire

Attach the red wire’s ring terminal to the front of the solenoid on the battery cable side.

Step 4: Connect Brown Battery Cable Wire

Attach the brown wire’s ring terminal to the bolt next to the red wire on the solenoid.

Organize Wires and Reconnect Battery

Now that the harness is connected, it’s time to go through and clean up the new wires with labels and electrical tape. We saved this step for the end to avoid incorrect groupings and to give ourselves the full length of the wires during the install.

Now would also be a good time to replace any zip ties with electrical tape to ensure nothing gets caught or pulled out of its socket. Also, now is a great time to address any other electrical issues in your classic Mustang.