1999 GMC Yukon Owner's Manual - Page 157

yellowblue To use the rear control, first turn the front control knob to REAR CNTL. Rear Heater (Without Rear Air Conditioning) (If Equipped) If you have a rear heater (without rear air conditioning), the control switch is located on the instrument panel. Then, the rear control can be used to increase and decrease the airflow. To increase and decrease the flow of heated air to the rear floor vents, move the switch marked REAR HEAT to the blower speed you want. The knob has 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