1997 GMC Suburban Owner's Manual - Page 155

Air Rear Heater (Without Rear Conditioning) (f I Equipped) If you have a rear heater (without rear air conditioning), the control switch is located on the instrument panel. To use the rear control, first turn the front control knob be to REAR CNTL. Then, the rear control can used to increase and decrease theairflow. To increase and decrease the flow heated air to the of rear floor vents,move the switch marked REAR HEAT to the blower speed you want. The knobhas three speed positions. To increase the flow of heated air, move the switch toward HIGH. To decrease the flow of heated air, move it toward LOW. To turn the fan off, move the switch to OFF. 3-5