Air Vents

Looks like they used a bunch of vacuum motors to control the air flow on your car. The one that is stuck in the windshield mode is the Panel-Defrost Door Vacuum Motor. Now, the whole system works off of the same vacuum source, so see if the outside recirc door is working. Usually when u are driving with the fan on low speed, and open this to outside air, there is an increase in flow. If you are not moving and running the fan on high, by closing the outside air it will sound like more air is flowing.
If this valve is working, then you know you have vacuum driving the system. Now you have to find out why that particular motor is not getting the vacuum or not working. If you dont get the outside sir thing to work, then you have a vacuum problem, and all of the vacuum motors dont work.