I am in a silimar situation. Basically the first time I had an issue with the light, I had to get a new mounting piece because it was cracked where you insert the bulb. Even after this change the light bulb only lasted for maybe 6mo and it died again. I replaced it 3 times since with a bulb from Radio Shack and it keeps burning out on me. I really dont understand, though I got so sick and tired I just screw it for a while until I want to take the time to mess with it. Its only the third brake light, not a pressing issue at least.

Not like the warped fan piece I have to tend with right now.

 

is there moisture getting around the bulb that could cause it!!

 

There wasnt any in my case. What I got from one of the GM Techs was that they have seen a problem with those lights due to the heat. Not sure if I can buy that, but it does almost constantly get direct sunlight daily.

 

Get fresh lamps, I would install all new since its a hassle to change them (test new ones first so you know they work). Use bulb grease (.99 at Autozone) on the contacts. It helps keep moisture out of the contacts. Wipe off the bulbs with rubbing alcohol and a cotton rag when you are done. It removes fingerprints and oils from your hands. It can help make them last longer. (Cleaning the bulbs applies to all lamps, even in your home.)