GM Service Manual Online
For 1990-2009 cars only

If the vehicle has the automatic dimming rearview mirror, it can be adjusted by holding the mirror in the center to move it up or down and side to side.

Press and hold the button, located on the mirror, for about three seconds to turn the automatic dimming feature on or off. The indicator light comes on when this feature is active. The automatic dimming feature turns on each time the vehicle is started.