GMC 2000 Sierra - Manual (Page 138)

yellow blue
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4. 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.
Outside Manual Adjust Mirrors
Adjust your outside mirrors so you can just see the side of
your vehicle and have a clear view of objects behind you.
Some mirrors can be folded in, to enter narrow doorways.
The use of hood - mounted air deflectors and add - on
convex mirror attachments may adversely affect
mirror performance.
Camper Type Outside Mirrors (If Equipped)
If your vehicle is equipped with camper type mirrors,
they can be adjusted so you can have a clear view of
objects behind you.
Pull out the mirror head to extend it for better visibility
when towing a trailer.