GMC 2000 Sierra - Manual (Page 135)

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To adjust for compass variance:
1. Press and hold the COMP button, located at the
bottom of the mirror, for three seconds until a zone
number appears in the display.
2. Find your current location and variance zone number
on the following zone map.
3. Press the COMP button on the bottom of the mirror
until the new zone number appears in the display.
After you stop pressing the button in, the display will
show a compass direction within a few seconds.
Electrochromic Inside Rearview Mirror
with Compass and Temperature Display
(If Equipped)
When on, an electrochromic mirror automatically dims
to the proper level to minimize glare from lights behind
you after dark.
The mirror also includes a display in the upper right
corner of the mirror face. This can be used as a compass
or to indicate outside air temperature.