GMC 2008 Envoy - Manual (Page 163)

If your vehicle has the rear seat audio system, the
lower buttons are used to adjust the rear seat climate
control system. The temperature of the air coming
through the rear outlets is determined by the front
passenger temperature setting.
P (On/Off): Press this button to turn the rear climate
controls on or off.
z9 y
(Fan): Press this button to adjust the fan
speed.
z\ y
(Mode): Press this button to change the
direction of airflow (panel, bi-level or floor) to the rear
seat area.
The rear control only turns on if the front climate control
system is on and not in defrost mode.
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