2003 GMC Sierra 1500 Pickup Owner's Manual - Page 140

Electrochromic Mirror Operation The electrochromic (self dimming) mirror function is turned on automatically each time the ignition is started. To operate the electrochromic mirror do the following: 1. Make sure the green indicator light, located to the left of the on/off button, is lit. If it's not, press and hold the on/off button for approximately six seconds until the green light comes on, indicating that the mirror is in electrochromic (self dimming) mode. 2. Turn off the electrochromic mirror function by pressing and holding the on/off button for approximately six seconds until the green indicator light turns off. To adjust for compass variance do the following: 1. Find your current location and variance zone number on the following zone map. Compass Variance The mirror is set in zone eight upon leaving the factory. It will be necessary to adjust the compass to compensate for compass variance if you live outside of zone eight. Under certain circumstances, as during a long distance cross-country trip, it will be necessary to adjust for compass variance. Compassvariance is the difference between earth's magnetic north and true geographic north. If not adjusted to account for compass variance, your compass could give false readings. 2. Press and hold the on/off button until a Z and a zone number appears in the display. The compass is now in zone mode. 2-53