Got a quick question cant figure it out. Its an 88 gmc. Turn the lights on, the tail lights light up but no brake lights. But when the lights are off and I hit the brake pedal they light up?

 

Make sure someone has not put a single filament bulb where a double needs to be.

 

All the bulbs are ok.

 

Those trucks had an issue with the circuit boards inside the taillights. You will need an extra hand (someone to press the brake) but I would unplug the light and test with a test light whether or not you are getting the right signal back there. If so you need the circuit boards, if not at least you have eliminated them as the culprit.