Defrost setting is default mode on our vacuum operated heater control system. If you have had any work done recently in engine compartment, you may want to check vacuum hoses at rear of engine compartment around the firewall.
Yeah, I agree with that. That's a good and important place to start, and an easy one to check. You definitely need vacuum supply to the vacuum swithing selector valve inside, in order to move ANY of the actuators from their default positions.
