GMC Envoy 2008 - brake lights won't come on

Where did you change the fuses at?, They are in the rear fuse block under the rear drivers side seat, not in engine compartment. Also, are you using LED bulbs?

Yes, I replaced the fuses in the block under the back seat. I haven't replaced the bulbs because the lights do work.

OK, 2 things. If you're using LED bulbs, I know your problem. If not, I'd replace the circuit boards the bulbs go into. On the back of the tailight, 3 screws to get to the bulb, that's the circuit board. Early on, there was actually a recall for these boards, don't think it's active anymore, but worth checking out, especially since there's no power getting to them.