1998 GMC Suburban Owner's Manual - Page 170

Your vehicle has air outlets in the center and on the sides of your instrument panel. You can move the outkts from side-to-side or up and down to directthe flow of air, or close the outlets altogether. When you close an outlet, it will increase the flow air corning out of any outlets of that are .open. These outlets do not completelyshut off airflow when in a dosed vent position. The use of hood air deflectors may adversely affect the performance of the heating and air conditioning system. Defogging and Defrosting On cool, humid days, use DEFOG to keep the windshieId and side windows clear. Use DEFROST to remove fog or ice from the windshield in extremely humid or cold conditions. Use DEFaQST with the temperature knob toward the red area and the fan control toward HI. 3-8