It sounds like the drain for the AC evaporator might be plugged. There is a drain pan that normally collects the condensation when the air conditioning is running but it's also possible for rain water to drip in there. There's a 4" rubber hose with a right angle bend hanging down from the passenger side of the firewall under the hood. Squeeze that tube to open it up and see if any water or debris comes out. You might also have to remove the hose and stick a pencil in there to dislodge anything.
Also look for dry white stains or green antifreeze dripping from that drain tube. Either one is a sign of coolant leaking from the heater core. Your observation that the coolant level hasn't gone done is a good one, but it doesn't take very much to fog the windshield. Coolant will leave an oily residue behind. Water that has collected in the drain pan can be vaporized and will be blown onto the windshield but it will evaporate cleanly with no residue. That will be worse when the windshield is cold and the incoming air from the heater is warm.
Very often a hole will corrode through the heater core but it will be plugged by sediment. That kind of leak can get much worse right after having the cooling system flushed. Mechanics get blamed for causing the leak, but in reality it was already there. That flush can prove the heater core needs to be replaced if the leak gets worse, but that isn't conclusive if the apparent leak stays the same. There still could be a tiny leak with no sediment blocking it.
Tuesday, April 12th, 2011 AT 6:15 AM