GMC Troubleshooting Questions? Ask a Mechanic for Answers
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These fuses you need to check are fuse 20 and fuse 28 in rear fuse box. If good next step is checking power/ground at the sunroof module. The orange wire should have power at all times, purple wire should have power with key on, and black/purple should have ground at all times. Here is a picture of module. Let me know how it goes or if you have questions, you can reply to this answer at any time without additional fees. Thanks.
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Circuit beaker 9 in rear fuse box is for the rear panel. Beyond that there isn't too much more you can do as this system is pretty complicated. The switch sends signal to the power roof module which sends signal to the endgate module which operates the rear panel. A scan tool will be needed at this point to monitor both modules to see which one is/isn't getting signal. Let me know if you have questions, thanks.