Air only flows through the defrost no matter what setting is used
The ventilation system is in default mode.
Do you have a digital readout for the heat A/C system or just knobs for controls?
Just knobs for controls
The modes are controlled by vacuum on this system, you need to check the source vacuum supply to the heater control that will be under the hood and come out of the firewall. If the source hose and supply is good then you will have to remove the heat/ radio cluster to gain access to the vacuum lines for mode control to test them, if they heck good them it is most likely that you have a failed vacuum servo motor that controls the door for mode control. Fords default setting is defrost if a vacuum leak or no vacuum is present.
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