Is there any indication of a mouse nest?...droppings, nesting materials, acorn shells, etc? I had a mouse problem and I now keep the air control on "recirculate" (middle button on far left control knob). I also smear Vicks VapoRub on the cabin filter every 6 months.
There were mouse droppings in the glove box and paper blew out of one of the vents (not the one that is blowing though) I have put it on recirculate, hot, cold, every setting there is at every different level but nothing has worked to allow the vent to blow. There is no smell of death so I'm not sure what to do and the Texas heat is not forgiving when missing even one vent.
Droppings in your glove box means mice are or have been in your A/C vent system. Check your cabin air filter. Some forum members have had mice chew it up to make a nest else where in the system. If a nest is blocking your vent it may be necessary to disassemble the dash area to clean out the nest.