As posted below, sounds like the post on the mixture door in the heater
air plenum. I had one break off on my Aerostar. Let us know if that
is what it proves to be. Since I replaced mine, I've been parking the
car with the door in the defrost position, while it is still warm. I
believe mine broke when adjustments were attempted while the car was
cold in freezing temperatures and the plastic was brittle. This seems
to be a common problem. Can anyone else attest to this part breaking
in unwarmed-up cars in freezing temperatures?