This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
Problem: under acceleration, air comes through the defroster vents
instead of the vents it's supposed to come through. Reason: a valve that
sits in a vacuum line went bad. That valve leads to a vacuum reserve
tank, and the tank either wasn't forming a vacuum or the vacuum in the
tank couldn't get out. (The reserve tank may also have gone bad, or
there may be a leak in the system). (Lloyd R. Parker)
Dan Stern noted there is a Chrysler TSB on this (24-06-92), which for
all but AB-bodies recommends replacing the vacuum check valve with one that
contains a reserve tank - part 4677204. (AB bodies use this check valve and
a couple of extra parts). Dan said this fix worked on his 1962 Valiant!
On many vehicles, this comes out of a vacuum hose which branches off the
power brake booster; it may look like a film canister or a small funnel. On
the Shadow/Sundance, it is underneath the dashboard and looks like a film
canister. This part is right on top and easy to replace. There is a
replacement check valve sold by Help! in auto parts stores but it does not
contain the reserve tank.
On my own 1991 Spirit, the rubber piece which the canister connects to was
also loose, leaking vacuum. ALL vacuum leaks are bad!