I have a 1989 760 with well over 200K miles on it. I just started having a strange problem with the driver-side brake light. This lamp has a dual-filament bulb serving both taillight and brake. I've replaced both this bulb and the passenger-side bulb a few times while troubleshooting this.

As long as the headlights are off (and the taillights are off) the brake light works fine.

If I turn on the headlights, the taillight will come on, but if I also press the brake while the taillight is on, it goes out entirely. The taillight will come back on when I let off the brake. The combination of the taillight and brake light causes the whole lamp to go out.

It gets more strange. If the brake light is on and I turn on the left turn signal (same side) then it will turn out the brake light as the turn signal lamp flashes on. So the brake light and turn signal alternately flash.

Summary, my driver's side brake light will not work at the same time as the taillight (in the same bulb) or the turn signal (on the same side). The passenger side works fine.

I've thoroughly checked all of the fuses and the bulbs. Tried pulling each bulb on both sides to see if they could be interfering, no luck. The bulb warning indicator seems to be working okay. It only comes on when that brake light goes dark. Any ideas?
I need to get this fixed immediately to pass inspection.

Thanks!
Henry

Check that the black wires have good ground contact

You have a bad ground. Check all grounds for the tailight assy.

Any time you have a strange unexplained problem like you described, it is a resilt of no or bad ground.

Wow, thanks for the consensus! I did check for good contact between the wiring and the assembly. How could I test for a good ground beyond that point?

Thanks again!