Having an issue with an 87 GT that I inherited..

3rd brake light works fine when driving
But when I turn on the running lights or headlights, the 3rd brake light turns on.
But it gets brighter when i hit the brakes, I am assuming there is a dual filament bulb in there for whatever reason

Also, when parked with the key off, the "Brake" light on the dash is illuminated and the car chimes until I take my foot off the brake, not sure if the issues are related.

Car is a 5 speed if that matters

I do not know if this will fix your particular situation, but its is worth a try....

There are certain things on the dash or with the dome lights that will get power, if there is a blown fuse. It bleeds back through the system somehow. I know that sounds crazy, but I DO remember it being discussed on here somewhere. I just am not sure if it effects the 3rd brake light or not.

Check fuses? Rear bulbs?
See my Cave, Sneak Path

Guess that rear bulb(s), Socket(s) or ground(s) for them are bad. 86/87 GT 3rd and rear bulbs use same lt blue wire but ground for 3rd is in the cabin.
Iffy rear ground then can sneak path taillight via lt blue and any ground path. sim to front sneak ground in cave.

Do not drive w/ this problem because 3rd brake can melt, maybe cause a fire, left on for hours. 889 is ~27w halogen bulb and gets very hot.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Not sure if these symptoms are related

https://www.youtube.com/watch?v=mLLGJ8kmKjw

I have an 87gt 5 speed as well that I just noticed does the same thing. Only thing I have done is put an led light in there. I will post up when j find what is causing it.

[This message has been edited by Spadesluck (edited 10-05-2017).]

quote Originally posted by theogre: Guess that rear bulb(s), Socket(s) or ground(s) for them are bad.

The rear harness ground is here:

quote Originally posted by Gall757: The rear harness ground is here: CLICK FOR FULL SIZE

Maybe but most times is iffy sockets and/or bulbs. Just 1 iffy socket/bulb can affect other bulbs.

I've tried to trace dash bulbs flash but don't see how that happens...
I would follow sneak path page above then look for other problems.
Example: If dash had work done... Plugs on back maybe not fully plugged in or have trace damage on the "board."

IF has LED then replace w/ normal good or new 2057 194 etc and check. Many LED have very low quality and cause problems.