GMC 1999 Suburban Owner's Manual page 158

Rear Air Conditioning and Rear Heater
(If Equipped)
If your vehicle has the rear air conditioning and rear
heater system combination, controls are provided to
regulate temperature, location and speed of the airflow.
To adjust the airflow speed, turn the fan control
knob on the left side of the control panel to the desired
blower setting.
To activate the rear control, move the fan knob on the
front control to REAR CNTL.
3-6
To regulate the airflow location, adjust the center knob
on the control panel. Turn the knob clockwise for floor
vent airflow or counterclockwise for headliner vent
airflow. Generally, the upper vents are used for air
conditioning and the floor vents for heating. The control
knob can be set to any blend setting.
To adjust the air temperature, turn the temperature knob
on the right of the control panel.
For warmer air, turn the knob clockwise to the red, and
for cooler air, turn the knob counterclockwise.
The rear control works just like the front control. It will
allow second seat passengers to adjust the controls as
they desire.
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