does sound your center vent is not closing/opening the flap for the vents....bet you have great defrost air! Mine did the same thing and to fix that seems to require removing the dash...I passed on that option and plugged the vac line to the pod, and super glued the flap so I always have center vent airflow....in short, I hacked it!

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Mark
1983 300TD Wagon
Even a broken watch is right twice a day