Same thing happened to me, I got a second hand blower off one of the guyz here! fixed the problem! Like rubjonny, it's the resistor that goes!!
Also i have the same problem as you with the drivers side window vent, seems to be coming through at a very slow speed, then the middle one next to it is blowing like a gale, i have tries pushing some rubber tubing down it to see if it is blocked but it seems ok! I am at a loss on this, maybe it just the way it is i don't know!!!!
If you find a cure please let me know!