GMC Troubleshooting Questions? Ask a Mechanic for Answers
Can you look under the dash and near the top of the brake pedal is the brake light switch. Look to be sure the wire connection is still connected. Sometimes it comes unplugged. If its still connected then the switch is the most likely cause. We replace quite a few of them.
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Ok, take a piece of wire or a paper clip and with the switch unplugged, jumper the wire with power, to the brown wire and see if the brake lights come on.
I dont have the wiring diagram in front of me and i thought it was the brown wire that completes the circuit to the brake lights but try jumpering the black/white wire with the power wire and see what you get. Have someone back to look at the lights and just touch the jumper for a second and see if they come on.
Darn, thats about as far as i can go without a wiring diagram so i will opt out and see if someone else can help. Best wishes!