1996 GMC Yukon Owner's Manual - Page 104

Compass Operation When the ignition is on and the mirror switch is in the C/M position, the compass display will show two character boxes for approximately two seconds. If, after two seconds, the display does not show a compass heading ("N" for North, for example),there may be a strong magnetic field interfering with the compass. Such interference may include magnetic antenna mounts, a magnetic note pad holder, or a similar magnetic item. If removing these items does not correct the condition, see your GM dealer for assistance. When cleaning the mirror, use a paper towel or similar material dampened with glass cleaner. Do not spray glass cleaner directly on the mirror as that may cause liquid cleaner to enter the mirror housing. Compass Variance Under certain circumstances, as during a long distance cross-country trip, it will be necessary to adjust the compass to compensate 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. Use the CAL switch located at the bottom of the mirror housing near the on/off switch. A safety pin or a paper clip can be used to press the button. Press and hold the CAL switch for five seconds until a zone number appears in the display. 2. Find your current location and variance zone number on the following zone map. 2-40