How to Install 5.0 Resto Headlight Relay Harness On A 1987-1993 Mustang

From the factory, there was a slight overload on the headlight circuit that, over time, can cause serious damage to the headlight switch connector that plugs into the back of the headlight switch due to the large amp draw. The connector can melt, which can cause damage to the switch, dash wiring, and intermittent headlight function. Well, thanks to 5.0 Resto, you no longer have to worry about that; the headlight relay harness will prevent this issue in the future. The 5.0 Resto headlight relay harness shifts the load away from that connector into two relays, one for the low beam and one for the high beams. Reducing amp draw anywhere from 5-10 amps per circuit greatly decreases the demand on the factory wiring. Regardless of whether you have good factory wiring, are doing a mild restoration, or are just working on making your Fox Body further updated, this harness is a fantastic option for your Mustang. We believe every single 1987-1993 Mustang needs this harness, as this is how it should have been wired from the factory.

Product Links:

• 87-93 Headlight Relay Harness
• Fox Body Mustang Exterior Lighting

Watch:

• Fox Body Front Bumper Removal
• Headlight Switch Replacement

Installation Instructions

Step 1: Remove The Front Bumper

• Start by removing the front bumper with headlights from the car. This step is not required, but it is highly recommended.

Step 2: Remove Battery & Tray

• Disconnect and remove the battery from the car.
• Next, remove the battery tray.

Step 3: Install Relay Harness

• Start with the starter solenoid. This is where the power is going to connect to on the relay harness.

• Connect the relay harness cable to the solenoid and zip tie the relay harness to an existing harness on the car that runs to the front end of the car.

• Once the harness exits the core support, mount the relays to the longer bolt that is securing the fender support bracket to the core support. 

• Connect the female connector the male connector on the factory harness. This is what was connected to the headlight bulb.
• Then connect the new male connector to the headlight bulb.

• Snake the ground wire up through the core support and attach it to the existing ground at the top of the core support.

• From there, take the harness across the core support and in front of the condenser. Use an extra hole to secure the harness with a zip tie. 

• Repeat the steps for the passenger side by snaking the ground wire through the core support, attaching it to the existing ground.
• Reinstall the battery tray and install the headlight bulbs before reconnecting the battery cables.
• Reconnect the battery cables to test for operation by cycling through all of the functions.
• After this, you are good to go. 

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