GMC 2003 Envoy XL Owner's Manual page 154

If your vehicle has the rear seat audio system, the lower
Outlet Adjustment
buttons are used to adjust the rear seat comfort
controls. The temperature of the air coming through the
rear outlets is determined by the front passenger's
temperature setting. The button with the up and down
arrows on it regulates the fan speed. The mode
button can be used to change the direction of airflow
(upper, bi-level or floor) to the rear seat area. To turn the
rear comfort controls off, press the lower PWR button.
Your vehicle may have rear seat outlets that can be
used to adjust the airflow toward either seating area,
the floor or upward. Move the control in the center
of each outlet in any direction to direct airflow.
3-29