I see...

For my diesel cars, I guess your gas car should be the same, there is a "pot" (vacuum actuator) behind the front console. It controls the opening and direction of the air flow (either to front-center or to defroster). There is a fuse to control the electronic parts for console. So, please check (1) fuse in engine compartment; (2) maybe the pot leaks.

Feed back with what you find out...