You are dealing with a vacuum leak problem. The vent doors are actuated by vacuum. If the loss of vacuum occurrs, you can't switch direction. Check under the dash for leaks. Also, with the engine running, you may get lucky and hear a hissing sound where the leak is located.
Let me know if you have other questions, and thanks for using 2carpros. Com. As far as a box, you would need to remove the entire HVAC system from under the dash.