I've been working on several projects on my 86SE. Rear Sway Bar, Rear Brake Calipers, and removing the flush-mount headlight system in favor of an 87 headlight system. The sway bar and brake calipers have been real ordeals.
This past Saturday, GFC member 2500fps and I got started on the job. I have a complete system from an 87 GT, so we started out with removing the hood as advised by many here and at Georgia Fieros. Loosened the fan shroud, radiator and AC condenser at the top then started at the firewall.
We couldn't get the harness end out without disconnecting a lot of things, including clutch and brake lines, so we peeled back the plastic harness cover so we could slide the wires through the small areas, a few at a time. In the course of doing this we found 4 or 5 places that had been wrapped with Duct Tape. We assumed this was some modifications that were needed to convert to the flush mounts.
Of course, as we got the new harness fitted into the front, we had to do the same to it to get the harness and connector back through the tight spaces of the master cylinder, brake booster, and clutch and brake lines. As we pulled the plastic wire loom cover off, we found the very same splices, each wrapped with silver Duct Tape. This has to be the way the factory put these things together. The 87 harness was original and untouched, as best as I know. No reason for anyone to splice wires together, exactly as the harness from the 86 was spliced and wrapped.
Once we got it installed, we found that the right headlight doesn't burn, and the left parking lamp doesn't burn. We never installed the left headlamp assembly because the right motor needs to be rebuilt. Might as well rebuild both before continuing.
Has anyone else found these same splices? They might be the cause of some of the front lighting problems, both in Generation 1 and 2 systems.
My findings this far on my 86 GT is that the fusible links are all wrapped with duct tape. But I had a hacked harness in my car from a previous owner who thought he could wire in two very simple toggle switches to flip the headlights up and down by bypassing the relays. What a lousy mess that was. I did not encounter issues pulling the old harness or installing the new one in the area of the master cylinder. But I did wind up pulling the hood off the car to properly place the harness where it extends to the passenger side. I could NOT find any other way to weasel it into proper place there.
Double check your grounds, including the headlight plugs themselves (these commonly have corrosion from lack of dielectric grease, and very often need the metal contacts repositioned so they will fully contact the three prongs at the back of the bulb. Use dielectric for every single connection and also inspect the side marker socket for corrosion/loose wires/bent contacts. If nothing obvious shows, do the dumb thing and turn the lights on while shaking/twisting/otherwise manipulating sockets to see if the bulbs flicker. Haha be careful.