No air out of vents blows out of window defrosts vent

Some vehicles control the vents with vacuum and when the system loses vacuum the system will send all of the airflow to the defrost vents. I think this is a safety issue, so that you will always have a clear windshield when the system fails. Check for vacuum leaks in the heater control system.

it could be the vent control vaccuum, it is located above the gas pedal to the right, it is metal, and there should be a bar comming out of it that moves up and down when you change the vent selection, it should be connected to a black lever, this controls the opening and closing of the upper and lower vents, it is made of plastic, and the end can get brittle and snap off the metal shaft, reapair it by drilling a hole in it and put a bolt thru it