GM Service Manual Online
For 1990-2009 cars only

The automatic day/night mirror uses two photocell sensors, located on the rear view mirror, to determine light conditions in the front and back of the vehicle. At night, as the glare of headlamps from behind increases, the mirror gradually becomes darker to absorb the glare. In the daytime, the mirror is in a clear state.

On some vehicle option levels, the left outside mirror also has glare reduction dimming that is controlled by the interior mirror.

Compass

The compass is an internal, non-servicable part of the automatic day/night mirror. It determines the direction that the car is heading by the Earth's magnetic field. The compass zone must match the zone in which the car is located for the compass to display the proper heading. Refer to Compass Variance Setup Description . The compass may also need calibrating. It has manual and automatic calibration. Refer to Compass Calibration Description .