1997 GMC Suburban Owner's Manual - Page 160

Defogging and Defrosting On cool, humid days, use DEFOG to keep the windshield and side windows clear. Use DEFROST to remove fog or ice from the windshield in extremely humid or cold conditions. Use DEFROST with the temperature knob toward the red area and the fan control toward HI. The defogger will shut itself off after several minutes. If you need additional warming time, press the button again. You can turn the defogger off at any time by pressing the button. Do not attach a temporary vehicle license, tape or decals across the defogger grid on the rear window. Rear Window Defogger (If Equipped) If you see lines running across the rear window, you have a rear window defogger. The lines warm the glass. To turn on the rear window defogger, press this button next to the fan control knob. The rear window defogger will only work if the ignition switch is turned to RUN. For best results, first clear the window of as much snow or ice as possible. NOTICE: Don't use a razor blade or something else sharp on the insideof the rear window.If you do, you could cut or damage the warming grid, and the repairs wouldn't be coveredby your warranty.