1999 GMC Suburban Owner's Manual - Page 156

yellowblue DEFROST: This setting directs air toward the windshield. To operate the rear air conditioning system, the front air conditioning system must be on. With the front air conditioning system off, the rear system controls can be used only to circulate air in the rear of the vehicle. Rear Air Conditioning and Heating Systems (If Equipped) If your vehicle has one of these systems, you can increase and decrease the airflow at the rear vents. Depending on the system you have and the setting selected, you can send cooled or heated air to the rear of the vehicle. Rear Air Conditioning (Without Rear Heater) (If Equipped) If your vehicle has rear air conditioning (without rear heater), the controls are located above the front and second seats. The front and rear overhead controls let you increase and decrease the airflow at the rear vents. To operate the rear system using the front control, just turn the knob to the blower position you want. 3-4