1996 GMC Sonoma Owner's Manual - Page 114

Heating The heater works best if you keep your windows closed while using it. On cold days, use HEATER with the temperature knob turned clockwise. BLEND is useful i n cool weather when you have fog or ice on the windshield or side windows. If you use the optional engine coolant heater before starting your engine, your heating system will produce warmer air faster to heat the passenger compartment in cold weather. See "Engine Coolant Heater" in the Index. Ventilation System For mild outside temperatures when little heating or cooling is needed, use VENT to direct outside air through your vehicle. Airflow is through the instrument panel outlets. Your vehicle's ventilation system supplies outside air to the inside of your vehicle when it is moving. When the vehicle is not moving, you can get outside air to flow through by selecting any mode and any f'an speed. You will find air outlets i n the center and on the sides of your instrument panel. You can move the outlets to direct the flow of air, or close the outlets altogether. When you close an outlet, it will increase the flow of air coming out of any outlets that are open.