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.
The Ogre's Fiero Cave