2005 GMC Yukon Owner's Manual - Page 179

On cool, but sunny days while using manual operation of the automatic system, use bi-level to deliver warm air to the floor and cooler air to the instrument panel outlets. To warm or cool the air delivered, turn the temperature knob to the desired setting. In AUTO mode the system will cool and dehumidify the air inside the vehicle. Also while in AUTO mode, the system will maximize its performance by using recirculation as necessary. Rear Window Defogger 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.