GMC 1999 Suburban Owner's Manual page 114

The mirror also includes an eight
in the upper right corner of the mirror face. When on,
the compass automatically calibrates as the vehicle
is driven.
2-44
-
point compass display
Mirror Operation
The right side of the switch located at the bottom of the
mirror turns the electrochromic mirror on and off. The
AUTO LED will come on when the electrochromic mirror
is turned on. To turn the mirror on, press and hold the
MIRROR button for three seconds. The mirror will darken
and remain dark until the button is released. To turn the
mirror off, press and release the MIRROR button.
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