It could be a bad earth or a wire has fractured is it on one side or bothe sides get a 12v electrical tester or a multimeter and see if you have current to the globe.
If you're positive it's not the bulbs (you know this because you checked them with a meter or tested the outlet with a test light maybe) or switch, check it out for a bad ground.
Bad bulbs or brake light switch.
Assuming you have already checked the bulbs and sockets, next check the fuse, then check the brake light switch, then check the signal switch.
Likely to be the brake light switch on the pedal at fault if the bulbs and fuses are good and other lights work ok.
The # is 1157 bulbs.
bulbs or brake light switch There is a recall on the break light assembly for the 05 Chevrolet Colorado. Check with a dealer for info.
you probably need to lift up on the brake pedal a couple of times to reset the brake switch. worked on our 86
Assuming you have checked the bulbs, check the brake light switch.
Change brake light bulbs Just remove the rear license plate. You can change the backup lights and brake light easily.
Check wiring for 12v also the brake switch on the brake pedal.
check that you have double filament bulbs in tail lights
Did you check the switch on the brake pedal assembly?