that air box isn't the easiest thing to figure out. More than likely the control motor for the door has failed. Search on heater core replacement - same steps needed to get the dash apart and remove the box. I'd then just test the 2 air box actuator motors and see which has failed...
I did my core a number of years ago -- pretty time consuming pulling the dash apart -- mostly everything has to come out (not fun). Figure 5-7 hours on the tear down...