'91 200Q: no air from dash vents

[Arun Rao <rao@pixar.com>]

I know this topic has come up many times on the list, but (of course) I
never paid attention.  My air conditioner is blowing air only through
the defroster vents and the footwell ... what's the common cause of this
problem?  I had a vague idea from my experiences with my '85 5KT that it
was a return spring on the blower box, but there is no such spring on
this car.

Any help is appreciated.

-Arun