Without being able to do a "hands on" troubleshooting of the circuit, no one can tell you the specific cause for this problem. So, the following "generic" answer is the best I can do to help.
There could be several causes, some of which that I can think of at the moment include:
There probably other reasons which do not come to my mind at this point. Good luck.
My rear window brake light works, but not my tail light brakes on my 1997 Monte Carlo. What is the problem?
the turn signal switch is probably bad
your brake light switch is probably out
If the center one works then it will almost certainly be 2 Blown globes.
you need to change the multifunctional switchthis happened to mine to
Check the bulbs one of the 2 filaments may be burned out
blown fuse or turn switch bad
its gotta be a ford a problem with the center brake light circuit causes the lights to go out i believe it is a diode
The left brake light works the right brake light doesn't and the bulbs is good
Try the turn signal switch in the steering column. I know it sound stupid but that is the problem and they are not cheap. At a service shop besides a dealer you are probably talking around $200+. I had the same problem, sometimes the rear brake lights would work and sometimes not but window light always worked. Good Luck
The ground for the brake lamp. When the tail lights are off, the tail light circuit can act as a ground for the brake light. When the tail lights are on, it can't.
Check bulbs?