It could be a minor mist leak at the heater core... poor you. I have been suspecting mine for two years but it's rearely a problem so I won;t go in there.

But it cuold also be a stuck recirc trap.
You can feel it if you stick your hand between the core and the firewall, remove the plastic passenger side cover at the consol...

Work the re-circ switch, make sure the trap opens and closes. If it's stuck closed, yu might just have an abundance of humidity in the car.
Often the hosing that goes to the intake (vacuum runs al lthe ducting valves as well as the recirc trap) deteriorates and looses pressure.

Greg Mustang
Montreal Canada