When you are up to it
remove your glove compartments
I won't swear to it, but you may be able to see the door that, allows outside air to enter the cab or close the door to keep the air outside.
If you can't see that door move when you push the recirculation button on the dash, then remove the fan behind the lower glove compartment door,
Then just up from the fan, you will see a little door, i think it just pops open. Behind that door is the Cabin Air Filter, if you have never changed it, you should.
Any way, with the CAF removed, look up through the fan hole, now push the recirculation button & hopefully watch the door move to the Open or Closed position. If it does not move, you may want to tape it closed until you can get it fixed