Sound's like a vacume problem. Ya see the hole vent system runs on vacum. Your lucky that it still blow air at all mine d/n work at all. I just haven't taken the time to take apart the dash to find the cracked hoses.
I had the same problem too - it just one day stopped blowing out the "defroster" vents - find your blower motor in the passenger wheel well, and see if the plastic panel thing that routes the air around is set right - mine wasn't, so it was just blowing air out the bottom.