1998 GMC Suburban Owner's Manual - Page 127

To adjust for compass variance: 1. Use the COMP button located at the bottom of the mirror. 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. 2. Press and hold the COMP button for three seconds until a zone number appears in the display. 3. Find your current location and variance zone number on the following zone map. 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 convex add-on mirror attachments may adversely affect mirror performance. 2-49