GMC 2004 Sierra 1500 Pickup Owner's Manual page 153

The compass can be calibrated by driving the vehicle in
circles at five mph (8 km/h) or less until the display
reads a direction.
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
zone eight. Under certain circumstances, as during
a long distance cross-country trip, it will be necessary to
adjust for compass variance. Compass variance 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.
To adjust for compass variance:
1. Find your current location and variance zone
number on the following zone map.
2. Press and hold the COMP button for five seconds
until a zone number appears in the display.
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, the display
will show a compass direction within a few seconds.
Passenger Air Bag Indicator
Your mirror may be equipped with a passenger air bag
indicator on the mirror glass, just above the buttons.
If you have this feature the mirror will display the word
ON, or an airbag symbol in Canada, when the
passenger air bag is enabled. For more information see
Passenger Sensing System on page 1-81
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