I just bought this '90 745 Turbo, and when I got it the lights were a mess. I spent today fixing it and this is the story so far:

RF headlight out - new bulb, fixed.

LR brake light, taillight out. Non-OEM taillight assembly partially melted, warped, etc. Lots of cleaning, scraping, bending, and adjusting later and the brake and taillight on the left side both work. 3rd brake light works perfect as well.

Bulb failure light blinking when right turn signal on. Cleaned the contacts for that bulb in the rear and no more failure light blinking. All turn signals work. The reverse lights work and the fogs work front and rear.

RR brake light out, taillight works. The bulbs are matched, the fuse is good, the bulb socket is good, the OEM Cibie light assembly is good and properly grounded. No brake light. The bulb failure light comes on when I hit the brakes. I switched the turn signal wire and brake light wire back there, and the brake light became the turn signal, but the turn signal light would not light up when I hit the brakes, so I'm 99% sure there's no zap going to that wire (it's the SOLID YELLOW wire).

The wire itself is in perfect shape back there at least. Where does this wire come from and where will I find the other end of it? What else could be causing this specific brake light to not work?