You are correct, Slappyn. Just like any electronic mirror, there is only one +12V wire to power the auto-dimming, compass and temp, though in order for the temp to display properly, you will need to get a temp sensor (installed near the front bumper) and run 2 lines (pos and neg) to the mirror from it. The rest of the pinouts on an OnStar mirror are for things you either don't need, or can't use. According to the diagram below, with your current harness facing you right side up (the tab should be at the top of the harness), the first wire furthest on the right will go to pin 13 of the OnStar harness ("on with ignition" power source), 2 will go to pin 8 (negative), and 3 will go to pin 9 (your "disable auto-dim in reverse" feature).
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