Another issues might be the resistor for the fan control, if it works only on high but nothing lower this is the cause.

BUT!
Can you hear the fan running or not? I guess we need to know this before adding anything else. if you canhear the fan, then it is something in the blend unit, I have seen ice form and sieze the blend door, burn out the blend door motor, and break the tab off the blend door activator.

It would help if we had a better description