I posted this a bit ago after I had the same problem:
My HVAC blower motor would run only on the high setting, and it was blowing weakly on the high setting. I had been putting off buying a new blower motor for probably the last year because it was still working, somewhat.
I read on this site about a "blower motor resistor" which is a crappy little piece that plugs into the blower motor. It's job is to prevent the heater motor from baking the ventilation system and causing a fire.
Anyway, I called the dealership and it was only a $15 part. It was simple to replace and now my A/C is running like a champ on all settings.
The only thing to keep in mind is that the blower motor itself can get clogged with leaf and debris and crap and can struggle to work, thereby causing a motor issue and possibly causing the resistor card to burn out.
I second the resistor card. Motor reistor card is on the evaporator case under the dashboard. You have to remove the glovebox and glovebox lining to get at her. It has a black plug and two screws, like 10mm or something.