100 window fogging

[JRued@aol.com]

my father owns a '89 100 which is fogging up the windows.  (Kind of like when
one goes parking for an extended period of time in cooler weather- though I
don't think this is the cause).  Heavy condensation is forming during drives
especially now that it is colder in the northern mid-east (Michigan).  I have
a feeling that the recirculate value (i don't know the official term) is
stuck and not allowing fresh air into the cabin.  He is too cheap to take it
into the dealer so he has been living with it for a while.  My question... is
any one familiar with this problem and how to fix it?  TIA  please e-mail me
at jrued@aol.com since I just subscribed and don't know when I will start
receiving digest.  thanks again... Joe