I have an '04 Trailblazer and there is a weird problem. There are three lights - top middle and the reverse on the bottom. The "tail lights", when the head lights are on, work fine. The middle lights (the brake lights) did not work. So I went and bought two new bulbs. I changed them and they still did not work. So I switched one in the middle to the top slot just to check. Now what happens is the two middle bulbs do not work - the top ones come on with the headlights and then only one - the top bulb - (passenger) illuminates with the brake depressed. But I believe the middle bulbs are supposed to do this and not the top bulbs. I believe the top ones are for the "tail lights" when the headlights are on. What is going on?
Put the bulbs back into the original position. The bulbs are different, one should have one filament inside and the other has two.
Check the owner's manual to see if you purchased the right bulb. Replace the bulbs if necessary.
If you still have the same problem, check to see if you have the proper voltage at the proper wires in the bulb socket.
If all voltages check out to be with in specs, then there is a few service bulletins and even a recall on the rear tail light assemblies and the internal circuit boards. They explain how vibration and moisture affect the proper operation of the lights.
The recall is NHTSA04v527000, it describes the problem and replacement of the tail light assemblies.
The other service bulletins explain the service and replacement of the circuit board in the tail light assemblies. The bulletin is 01-08-42-001H.
Check with your local dealer to see if your vehicle qualifies for these service bulletins.
If you need the full service bulletin and recall you can get it at ALLDATAdiy.com.