Replacing Your Front Wiring Harness

The article featured on this page is from the June 2011 issue of Auto Enthusiast Magazine.

Replacing a GTO front lighting harness

Story Jim McGowan

I replaced the original engine wiring harness in my ’70 GTO Judge with an exact duplicate and was very pleased with the results. Now it’s time to replace the front lighting harness with a new one as well.

The best part about these harnesses is they fit perfectly and use all the same connectors as the originals from GM. No cutting, soldering or splicing is required. Both of these harnesses are removed and replaced almost exactly, depending on your options, for the 1968 through 1972 GTO, LeMans or Tempest. There is a difference between Ram Air and non-Ram Air harnesses, so specify your engine and options when ordering.

The lighting harness is a little more difficult to install, as it must be snaked through some tight places. Obviously, it would be much easier with the urethane nose off the car, but that is really a pain. The front end on this car is lined up too nicely to fool with it. So, I will install the harness with the nose in place.

The degree of difficulty is not that great if you simply take your time. I took about an hour and figured the best way to approach the removal and replacement. By fitting the components on the new harness through those tight places to make sure they would clear, a lot of wasted time and headaches were avoided.

Disconnect and remove the battery; you’ll need that area empty to work with the wiring. To access the front work area, you will have to remove the filler panel at the top, the front grilles, the headlights and turn signal lights. All this took about an hour. No special tools are necessary and small hands are a plus. I started by unplugging all the wire connectors from the bulbs and side marker lights.

There are also two ground wires under each front fender at the core support. These are a little tough to access but you can do it! With all the connectors free, I pulled the wiring through an area at the bottom of the nose-mounting bracket. The three-prong headlight connector has to be worked through but the rest simply come right out. Then disconnect the horns. The harness in this car was lying at the bottom of the radiator instead of in its original holders at the top of the core support. After a repaint years ago, the body man neglected to put the harness back in the clips while replacing the nose. Rather than fight all that wiring through the second nose support and up through the core support, I cut it just before it passed through the support. Then it comes out through the grille opening.

After disconnecting the second set of ground wires under the driver’s side fender, removing a plastic plate in the core support and unplugging all the connectors, the rest of the harness can be pulled up from under the fender and through a rectangular hole in the support. The photos show this access hole.

Now to the bulkhead plug. Loosen the 3/8-inch bolt securing the large wiring plug to the firewall and pull the plug out. In this case, the engine wiring plug is new, and the lighting plug is original. The lighting plug slides out of the larger engine plug. Disconnect the wiring from the horn relay and unplug the wire at the brake distribution block on the frame. Now you can remove what’s left of the old lighting harness.


Ready to Re-wire

I started the install at the same place I started the removal, at the passenger side of the nose. I snaked the new wiring and connectors through the nose support bracket from the empty grille hole, reached through the directional light hole to get them, and let them hang out the light sockets. Then I placed the new wiring into the factory holding clips working toward the center support. The two ground wires need to be pulled up through the hole on the core support but there is no way to reach that high through the turn signal light opening. I used a test wire with alligator clips at each end to do this. I fed the wire through the access hole from the engine side, and connected the clip to the ground wires ends. Then, I carefully pulled the wires up and through the hole to be reconnected. This worked well.

Moving toward the driver’s side, I first wrangled the bulkhead plug, still wrapped in plastic, through the space at the nose support bracket. This took a little doing, but it went through after a few tries. Then the rest of the connectors were pushed/pulled through. Using a little body English, I reached under the fender while pushing the bulkhead plug up with my arm in the directional light hole. This worked just fine and I’m only on cut number five. Ya gotta bleed a little! Pull the plug and ground wiring into the engine compartment and set it aside for the moment.

Now connect all the bulbs, etc., on both sides to the harness and reinstall the headlight bulbs with the retaining rings, directional lights, etc. Then reconnect the two ground wires under the driver’s side fender and move back to the firewall plug. Install the lighting plug into the engine harness plug and reattach to the firewall. Don’t overtighten the 3/8-inch bolt or you might break the plug case; just seat it to the firewall snugly. Then connect the two wires to the horn relay. They will only go on the correct terminals, so no worries. Reconnect the wire to the brake fluid distribution block. Now you can reinstall the battery and connect the cables.


Here’s where I ran into trouble

All the lights and horn worked perfectly except the directional signals. NO light there at all. I checked power at the socket and it was fine. So I compared the original socket to the new socket and found the problem. The original sockets have two connector spades in each that must be removed and transferred to the new sockets. They simply pull out and push in. After I did this, both directional lights fired up and everything worked. The removal and reinstallation took about four hours working slowly as to not amputate any body parts. The new harnesses look great and fit perfectly.

Since a picture saves a thousand words, check the photo sequence and you’ll find a few tips that will make the job easier.


1 This is the new headlight harness. The bulkhead plug is packed with electrical grease so it comes covered in a plastic bag. Leave the bag on the harness until you’re ready to connect to the firewall.

2 Remove the front radiator filler panel to access the grille retaining bolts. The grilles must be removed to move the harness parts across the front of the car and fit into the original retaining clips. There are four 3/8-inch screws holding each front grille on the ’70 model GTO. You’ll find two at the bottom and two at the top.

3 This is looking at the driver’s side urethane nose support. Notice at the bottom you can see how the wire passes through the curved opening. This is where the large firewall plug and other connectors will pass during the harness installation.

4 Under the driver’s side front fender at the core support is this block off plate, with a slit in one side that the harness passes through. Carefully remove it, as it might be brittle. This one was still very pliable. You can see the large hole in the core support with the old harness still in place.

5 This is the access area in the passenger side of the nose-mounting bracket. I’m pulling the connectors through that hole without any problem. There are a lot of sharp edges on all these metal parts, so beware.

6 Here’s the original harness after I cut it to facilitate removal. I never throw out wiring like this, as all the connectors could be used in a pinch. Bag it and stash it.

7 The new three-prong headlight plug can be cantankerous. It will fit through the opening with a little patience. You can reach through the turn signal lamp hole to work it from both sides.

8 The firewall plug is now detached and the lighting harness plug removed from the engine harness plug. This requires a 3/8-inch small socket for removal.

9 In order to snake the two passenger side ground wires through the hole in the core support I used a wire with two alligator clips. Push it through from the engine side and reach in and attach it to the ground wire connectors. Then pull up through the hole.

10 Here is the new harness in the original factory retaining clip. There is plenty of slack available in the harness so you don’t have to pull it tight. If any clips are missing, finding the correct clips and using them will ensure a reliable harness.

11 By reaching through the turn signal lamp hole, you can feed the firewall plug up to the hole in the core support and grab it yourself or have an assistant pull it through. The contact grease is messy; that’s why the bag was left in place. The block off plate and ground wiring can now be put back in place. I cleaned the metal where the ground wires mount for a better contact surface.

12 This is the complete firewall plug connector with both the engine and front lighting harness together. It can now be pushed back onto the firewall connector and snugly secured with the retaining bolt. A pair of thin rubber gloves will keep the grease off your hands.

13 Here we see the harness running along the inner fender well and all the wires attached to the horn relay and down to the brake fluid block. The installation is finished.

14 Since you’ve already got all the bright work off the front end, it’s a great time to polish and detail the items. Also clean any bugs, etc. from the front of the radiator.

15 Here’s a tip. Wrap a piece of tape around the head of the headlight trim ring screws and they will be a lot easier to reinstall.

16 I installed the battery and tried all the front lights before reinstalling the trim. Everything worked fine except the directional signals. There is power to the harness, so what’s the problem? The original harness plug is on the right. Notice the spade connectors in the original. They must be removed and inserted into the new connector. Once I did that the directional lights functioned normally. This is another reason for not throwing out the old wiring.