I hate to say it, but it is most likely your heater core. It might be worth trying the black pepper trick to see if it clears up (after you have eliminated other possible sources of moisture). Early this year I had a heater core with such a tiny pinhole leak that it sealed itself back up before I removed it. It took a while to find, and I think that the old black pepper trick would have actually kept it from steaming my windows for a long time (although I am glad I fixed it).

I hope for your sake that it is not the heater core (for instance, if the heater box drain is clogged, you might get similar symptoms) but if it is, search for my post several months back; I went 95% of the way to removing the heater box from the car and ultimately decided that cutting it could be just as sound a repair. That way, you only have to remove the dash pad and a few other pieces.

Good luck. Watch your coolant level!!!