I have a 2006 Chevy Trailblazer LT 4WD. My brake lights do not come on. The roof mounted tailight does come on. I have replaced the bulbs, checked the stop fuse (#12 in the engine compartment), and replaced the brake light switch on the pedal. No luck.
If your high mount was coming on, then your switch was good. Included is a schematic, check first that fuse 34(VEH STOP) in the rear block is good, and that there is power across it. Then move out to the rear lamp connectors, and that switched power and ground is available at the circuit boards.