Welcome,The controls that change the air flow direction are vacuum operated and the most common thing that happens is that it looses vacuum because of a leak in one of the small vacuum lines. If you look on the driver side inner fender you will see the vacuum storage container where vacuum is stored. Follow the vacuum lines from it, to where it goes through the dash to the controls and flow it from the storage tank to the engine. You will probably find a cracked/broken/disconnected small plastic vacuum line.If the vacuum lines all look good and there is vacuum going into the controls then let me know and we can go further.
Click here for diagram, and here.
The hoses looked & fealt ok. I disconeted a red & blue hose lines from a canester near the gas peddle didn't feel any vacumn. It can be intermited at times. this time when turning the switch to floor or defrost air would flow to them independly & not the dash. Restared engine a few minnets later & control would go to the floor & not defrost just keep coming out the floor.