2003 Ford Taurus, 49,000 miles

I have a problem with my air conditioner/ heater, this problem started around about Sept. 2005. First there was a squeeling noise coming from the vents, but everything was working fine. Then after some time, the vents stopped blowing air except for when it was in the highest blowing strength. I took it to a mechanic and they replaced the resistance coils and everything seemed to be fine. But now more time has passed and now the vents don't blow anything out at all. Could this be the blower motor? If not, what else could it be and how much would it cost me? Thanks!