WD40 all of the sliding/moving parts connected to the vent door by the passengers feet. cycle it a couple of times. if it does not move, it is either corroded in the open position or the control cable has come loose.
TREAT EVERY STRESSFUL SITUATION AS A DOG WOULD.
IF YOU CAN'T EAT IT OR PLAY WITH IT,
JUST PEE ON IT AND WALK AWAY.