I have a 2001 Expedition that the dial switch for the heater/ac is not working. No matter how you turn the switch the air blowes up thru the defroster. The off poition on the switch works properly all of the other setting directs air thru the defroster. I also noticed when the key is on there is a rapid clicking noice coming out of the floor vents.

 

Suggest tracing the black vacuum line near the heater core where it goes thru the firewall. All sorts of posts here about it breaking at the firewall on the engine side. The heater systems defaults to defrost with no vacuum. Safety design in case of failure. Better to have air in your face than a fogged up windshield.

 

That was it. It is a very small vacuum line right behind the battery. Very easy fix.