The 2004 trailblazer had a factory recall for this problem, all the brake lights (except the third high brake light) would burn out early. Call your Chevy deal and see if your vehicle is part of this recall. If so, they will perform a 15 minute procedure to change a some module and you will be back on the road.
It is free since it is a recall.
Hope this solves your problem
Saturday, January 26th, 2008 AT 9:35 PM