Q: brake light problems on 2002 GMC Savana 1500

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when I turn the lights on the top and bottom bulbs come on, when I step on the brake the bottom bulb goes out. (the 3rd brake light works normally all the time)

when the lights are off and I step on brake the bottom bulbs come, they are dim though.

I can pull the fuse and everything still works as described. I believe its a ground problem, just not sure where to start.
(2) Answers
I had the same problem with my 2001 Savana. I found that the 2 sockets for the brake lights were bad. The GM dealer wanted about $110 each side for the rear wiring harnesses’. I found a replacement socket at the local auto parts store for $16 each. They look like the originals but take the rectangular base bulbs. It took about 45 minutes to replace both sides.
We are having a similar problem, where everything is fine except the passenger side rear light. When the brakes are pressed, it tries to light the marker filament instead of the brake filament (resulting in a very dim brake light when the headlights are off, and no light if the headlights are on. So my question to you is, was this a used van that was previously used by Comcast?