GMC Envoy 2003 Owner's Manual page 150

The left button with the up and down arrows on it
regulates the fan speed. The right button can be used
to change the direction of airflow (upper, bi-level or floor)
to the rear seat area. To turn the rear climate controls
off, press the middle button.
3-30
Outlet Adjustment
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.