It operates by an electric motor that moves a flap that directs air through
whichever vent position you have selected. The master flap is located under
the center of the dash in the middle and you can feel it if you reach your
fingers up through the floor air vents.
If you are lucky, it is nothing more than the flap edge stuck to the wall of
the vent and won't move to open up and send air to the defroster vents. I
can remember cleaning mine several times because the rubber edge of the flap
gets sticky with age. If you are unlucky it is a burned out servo motor
that moves the flap or a stripped gear and will need to be replaced. The
motor and gears are located up under the dash on the passengers side of the
center air box. You should be able to find a replacement easily enough in a
junk yard for only a few bucks as these spanned many model years.
Cheers - Jonathan
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