GMC 1999 Suburban Owner's Manual page 157

To use the rear control, first turn the front control knob
to REAR CNTL.
Then, the rear control can be used to increase and
decrease the airflow.
Rear Heater (Without Rear Air Conditioning)
(If Equipped)
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.
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If you have a rear heater
(without rear air
conditioning), the control
switch is located on the
instrument panel.
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