GMC Troubleshooting Questions? Ask a Mechanic for Answers
Does the right rear turn signal still operate?
Looks like all of the rear lighting shares a common ground, so not likely that a bad ground would affect just one light. If you have access to a test light or voltmeter, have someone press the brake pedal while you test voltage on the light blue wire at taillight connector on right side. Let me know what you find there.
Sounds good. I will look forward to your response.