I believe you have a stuck vacuum actuator.When you switch between vent, defrost, floor, etc there is a vacuum line that activates an "arm" that moves a director that channels the air to the proper vents. You either have no vacuum to the module or the module is stuck.You may be able to see this module by removing the glove compartment box. Usually it is next to the blower motor which always sits behind the glove compartment box. Try moving the arm around and getting it to unstick.