Three guesses, and only guesses.
1: what John said
2: Trailer towing wiring
3: supplemental ground to fix some long ago problem.
The factory never used those terminals but with the age of the car, who knows what some mechanic did somewhere. He may have fixed some symptom but not the problem.
As previously mentioned, the brake lights go through the turn signal and four way flasher switch and I would look there for what may be going on in your car.
Craig