When I first noticed that my left brake light wasn't working it was nite time and my head lights were on. So the next day I changed the bulb, and thought that it was working just find, but when I turned the headlights on the brake light went out. The light will flash when I put the left blinker on, but If I use the right blinker then it goes out. It does work when the head lights are off.
Could the problem be a blown fuse?
or something more complicated that has to do with the light system and blickers?

It sounds like you have a bad ground at the tail lights or a bad light socket.

What Choo choo said & make sure the bulb is making good contact.

One other thing you may want to check is your light bulb. You can very easily install the bulb backwards in the socket. GM's light sockets in those years were not of the best quality and it's easy to install the bulbs with the side pins in the wrong slots. I have ran accross this many times in the past. Just a thought.....

thanks for the help. I'll post back when I see if what you guys said works.