GM-GMC: Heating and Air Conditioning, chevy silverado, 2003 chevy silverado

Question
I have a 2003 Chevy Silverado 1500. When I turn on the blower for heat or air conditioner, the air will blower through the floor vent, then the facial vents and finally through the defrost vents. It will stay on defrost and I cannot change the direction of flow with the selector switch. What is the cause of this condition and how can I fix it?

Answer
The defrost position is a default position, so your windshield will be clear if something does go wrong with your hvac system.  Typically this is due to one or two things.  The first is that your control head is malfunctioning, not sending the proper, or any, signal to the actuators to change position.  Depending on the system, some are digital some are a simpler electronic version.  The second reason, and usually more common is that the actuator for the defrost has been unplugged on has fallen off.  It is located in the centre under the dashboard.  It is relatively easy to locate, Located on the 'hump' so to speak directly below the ash tray etc.  It will be mounted to the hvac duct and is approx the size of a dech of cards and should have a three or four wire connector going to it.  Have a look at it and see what you can see.  You should be able to watch it move as the hvac is turned on.
Hope this helps.