The selector switch for defrost,floor, vent will not switch air flow.

Is this a manual control? If so then the cable, control or bell-crank may have failed. The bell-crank is on the passengers side of the plenumn is visible with the glove box, and lower dash panel removed. Start by checking to see if the control assembly moves the cable, and if the cable is still connected to the linkage (bell-crank).