2004 GMC Yukon Owner's Manual - Page 190

Defogging and Defrosting You can use either defog or front defrost to clear fog or frost from your windshield. Use the defog mode to clear the windows of fog or moisture. Use the front defrost button to defrost the front windshield. Rear Window Defogger If your vehicle has this feature, the lines you see on the rear window warm the glass. The rear window defogger uses a warming grid to remove fog from the rear window. -(Defog): Use this setting to clear the windows of fog or moisture. Press the mode button to select this setting. This setting will deliver air to the floor and windshield outlets. 0 (Front Defrost): Press the front defrost button to defrost the windshield. The system will automatically control the fan speed if you select defrost from AUTO mode. If the outside temperature is 40°F (4°C) or warmer, your air conditioning compressor will automatically run to help dehumidify the air and dry the windshield.