What would cause the main brake lights (tailights) on a 2000 Chevy Silverado not to work while the cab brake lights work?

I would start by checking to see if there is power at the bulb ( ie. testlight). If not start inspecting the wiring harness and quick disconnect plugs located under the truck bed along the frame, these can be traced from the tail lights back. It could even be a broken wire in the harness.

Check the brake light switch underneath the dash...it has a button that works off the brake pedal arm