1989 Ford Aerostar Possible exhaust leak

On rather wet weather the windshield gets all fogged up when you turn the defroster on. There appears to be something like smoke coming out from under the passenger side. I really can't tell what it smells like but if you do not roll down the window it is hard to breath, but the smoke(or whatever it is) still is there. Could it be the exhaust or a part on the heater core that would cause this problem. This just started about 3 weeks ago.
Please help.