It could be anything from a broken blend air door shaft, a blend air motor
that isn't working, the control head may be burnt out. A scan tool to see
the inputs and outputs of everything may be needed to verify and to retrieve
any fault codes stored

Glenn Beasley
Chrysler Tech

I have a 96 Grand Caravan 3.3 liter v6 with exactly the same problem.
I did the code check without reader
http://www.allpar.com/fix/80s-codes.html  but nothing showed up.  Maybe
a reader checks more?   I can hear a motor/acuater trying to do
something when I operate passenger side heat control.  I started to
disasemble dash but decided to do a little more research and possibly
buy a repair manual.  Dealer does not stock the blend door shaft if
that's what it is.  The passenger side blend door actuater seems to be
on right side, but I'm not sure.  If I have to pay $950 to repair the
heater on a $3,000 van I'll dump it and get something more reliable.  I
have a few other electrical problems, intermitent left turn signal,
rear wiper will sometimes start on it's own, small agrivations, but
since I'm thinking of driving north this winter the heat issue is
serious.  Already put $600 out for a transmission seal on a tranny that
had just been rebuilt by previous owner.  I really miss my 93 Voyager 4
cyl 5 spd, good milage no problems except axel seals.   A old Ford van
pulled in front of me and it was totaled.