GMC 2003 Safari Owner's Manual page 147

Outlet Adjustment
Move the thumbwheel in the center of the outlets up or
down to change the direction of airflow.
Operation Tips
1. Clear away any ice, snow or leaves from the
air inlets at the base of the windshield that
may block the flow of air into your vehicle.
2. Use of non-GM approved hood deflectors may
adversely affect the performance of the system.
3. Keep the path under the front seats clear of
objects to help circulate the air inside of your
vehicle more effectively.
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