Brake Light Problem

I have a 95 Buick Regal Custom 3.8 and my brake lights work sometimes and sometimes they wont. Sometimes 1 is out, sometimes just the middle one works. Just this morning the passenger side was out and the middle and the drivers side were working, now they all burn. What could cause this. I have replaced the switch and all bulbs.

i would check the wiring in the trunk make sure there are no loose connections, if the middle one is always uneffected then the problem is with the wiring behind the main lights

Forgot to mention that sometimes the middle one is out too. No brake lights at all.

I have a 96 Regal Custom with the same brake light problem, the problem may be in you turn signal switch... try pulling back on your turn signal lever slightly, not enough to trigger the high beams, while your foot is on the brake, if your problem is the same as mine, your brake lights will work correctly while applying slight pressure to the turn signal lever... picked up a turn signal switch from a junk yard for ten bucks... problem solved... word of caution, follow the safety precautions concerning the air bag in the steering wheel.