A quick fix is to remove the upper grill, cover the main air intake with
cardboard and plenty of duct tape.  (Note to all literal readers:  Replace
grill when air intake is covered.)   This still allows some fresh air to
enter through the two small side air intakes which are on either side of the
main air intake.

Next spring, remove the cardboard and duct tape.  However, this fix will
also keep out much of the hot, humid air during a/c season, so it may be
worth keeping in place.

The proper remedy is to replace/repair all of the offending (no longer
effective) seals in the fresh air intake system, but this is a real PITA.

You may also find that the hose leading from the front air intake to the
side dash vents has come loose, which renders the dash vent flap useless at
closing off the fresh air, in which case it is simple to re-attach the hose.
However, if this is the cause, the cold air tends to blow in from under the
dash, rather than through the dash vent itself.

Budd Premack
86 Syncro
Minneapolis, MN (Land of Sky-Blue, yet almost frozen, Waters)