Ok I can't make any sense of how this would even be possible but on my 01 I have 1 single solitary vent on my dash that won't blow air. The passenger side vent closest to the door has no airflow while the other 3 on the dash do. My defrost vents and floor vents work fine. Just this one vent has no air. How is his possible ?? What do I need to look at and How do I get to whatever it is without wrecking the dash ??

 

Since I did a 97 Mustang dash swap into a 84 Tbird I can tell you that the plastic vent tube runs from the airbox behind the middle of the dash, then under the passenger side airbag and over to the passenger side air vent register. Bet's are on that this tube slipped off at one of the ends. Check behind the passenger side vent first since it is the easiest to get to. If that is secure then perhaps it has slipped off the central airbox in the middle of the dash. Removing the glovebox completely will assist in accessing and viewing the vent tube.

There is nothing in the central airbox that individually regulates the airflow going to the passenger side. However there is the obvious thumbwheel on the vent itself that regulates opening and closing the vent.