2003 GMC Sonoma Owner's Manual - Page 150

Outlet Adjustment The flow through outlets on your vehicle allow outside air to enter your vehicle while it is moving. Outside air will also enter your vehicle while the air-conditioning compressor is running. Operation Tips 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. Use of non-GM approved hood deflectors may adversely affect the performance of the system. Keep the path under the front seats clear of objects to help circulate the air inside of your vehicle more effectively. Use the thumbwheels located in the center and on the sides, to change the direction of the air flowing through the outlets. 3-23