I had the exact same problem, in 1996 Plymouth Voyager.
It always happened after a rain.
This resolved the issue for me:
1. Open hood and look for a hose on the passenger side, underneath the wind shield wipers.
2. Disconnect the hose (mine just slid off).
3. Check the nozzle that the hose was connected to, and clear it of any leaves or trash.
I believe that a vent o circulate outside air into the cabin,
is designed to let rain water flow out thru that hose.
When it is clogged, rain water fills the vent.
Tuesday, August 28th, 2012 AT 5:22 PM