why does air only exit through the defroster vent

What is your question?

no matter what operation I select , air only exits through the defroster vent. When I switch to air conditioning the cool air only exits through the defroster vent.

What makes this problem better or worse?

used to switch vents from dash vents to defroster vents and back, now only defroster vent

How long has your 2003 Mercury Sable had this problem?

about 6 months