Suburban (2002) 3rd brake light problem

I have a 2002 Suburban 3rd brake light problem. The normal break lights work OK, the 3rd (highmount) break light does not work when I shove down on the break pedal. However, if I manualy slide (turn on) the electronic trailer break and the 3rd break light and lower break lights all come on, so the light is getting power. I disconnected the electronic trailer break controller and I still have the same problem. I checked all of the possible fuses and they seem OK. I spent about ten minutes watching other vehicles (Surburans) at a stop light and notice several of them had the same problem, the bottom break lights work but the 3rd break light does not. Anyone have any suggestions?

Thanks!

Jefftex
:banghead: