I just fixed a problem that sounds similar to yours, however my tail lights were working - just the backup lights were not. I checked the bulbs, fuses and wiring, and all were fine. The solution was to replace the backup light switch on the transmission. The switch is on top of the transmission near the clutch cable. There are only 2 bolts attaching it, but they're a little hard to get at. Since the switch is $20 - $40 depending on where you get it, you might want to remove the switch and test it with a continuity tester first to make sure it's the cause of your problem. You can connect the test leads to the switch terminals and operate the 3 different switches on the unit to see if they're working. I think there's 5 terminals, so you might have to do some trial and error to find which terminals go with which switches. Mine was only working partially - I'd push the switch and see continuity at first, but then it would open up again as the switch travelled to what should be its fully-closed position. Just a note: there seem to be 2 different switches available - one with, and one without, an "upshift indicator" function. My dashboard has the upshift LED indicator on it, so I purchased the appropriate backup light switch. The backup lights now work, but the upshift indicator still does not. ? Haven't solved that one yet, so any help would be appreciated. !
I had what sounds like the exact thing happen to my 87'Cabriolet. All I had to do was change the "on-off light switch". It took me about 10 minutes and I'm a girl!! I have never had the same problem since. I hope it is as easy for you. Good Luck!
the turn signal switch is probably bad
If you have the same bulb for brake and signal, brake light power needs to be interrupted by the signal lights, The brake circuit has to go through the signal light switch so the problem could be there.
Fuse
fault brake light switch, located at the top of the brake pedal.
Since most brake/turn use the same filament in the bulb(1157) then if the turn signal switch is bad or offset then it will effect the brake lights.
Check and make sure you have the correct bulb. The turn signal/brake light is a dual element bulb and if you have a single element it does not work properly.
I would check the running lights for one with a short. I had the same sort of thing with my trailer and I spent way to much time troubleshooting the brake and signal wires. Turns out a running light bulb connector was grounding out on the trailer and messing it all up.
Tail lights. Brake lights. Backup lights. Signal lights. License plate lights.
either dimmer switch or blown fuse If you are saying that the tail brake lights are working but the third brake light is not....It may be a defective turn signal switch in the steering column.
Assuming the signal lights still work, no. But a bad brake light switch or a blown brake light fuse could.
One of two things. Either the brake light switch behind the brake pedal. Or try wiggling or rotating your turn signal stalk in a circle motion. If the brake lights work when the turn signal stalk is wiggled then you need to replace the turn signal stalk.
Check the brake light fuse, the bulbs, the brake light switch and last, check the signal switch.