is the heat coming out of the vents hot? no heat at all will fog window pretty fast in the cold air.

if its hot id really lean in the heater core as the problem. they suck to change honestly,( but really it could be worse) but doing it your self is well worth it vs the price a shop will rape you with.

i will cost ya a afternoon. or less depending on your skill level. lots of small bolts to get to.