Found this on another forum for Compass calibration
Once the mirror is connected and powered up the compass display will read "C" or "CAL" because a long term loss of power allows the mirror to lose its setting of where north is and needs to be re-calibrated. If your mirror powers up and displays a heading, chances are its wrong and you need to force the mirror into calibration mode. To do this hold the "comp" button down for approximately 6-10 seconds or until it reads "C" or "CAL" then proceed to the next step.
If your compass is displaying "C" or "CAL", simply drive in a tight circle (in a parking lot for example) under five miles an hour until a compass heading is displayed. Typically this should take less than three complete turns. If this doesn't work, drive normally and over the course of your route a heading should display.
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