Mixer door, there is a little metal wire with a loop in it that hooks to the mixer door, up under the dash in the passenger side, kind of behind the console / radio / hvac controls. Check to make sure it is hooked to the door mechanism, and the mechanism is moving the door when you turn your temp knob. I'd bet it slipped off or one of the plastic arms snapped.
Hard to explain exactly where it is.. if you sit in the passenger seat, it is to the left side above the toes on your left foot.