GMC 2002 Yukon XL Owner's Manual page 190

Rear Air Conditioning (Without Rear Heater)
(If Equipped)
Front Control
To increase and decrease the flow of cooled air to the
rear vents, turn the knob to the fan speed you want.
The knob has three speed positions. To increase the flow
of cooled air, turn the knob toward 3. To decrease the
flow of cooled air, turn it toward 1. To turn the fan off,
turn the knob to OFF.
3-10
To operate the rear system using the front control, just
turn the knob to the fan position you want.
Rear Control
To use the rear control, first turn the front control to
REAR. Then, the rear control can be used to increase
and decrease the airflow.