What is the problem ?
When I used my aircondition my blower fan work only on high on 2000 Plymouth Breeze
The switch needs to be replaced.
I have to disagree a Lil. I'd start with the blower resistor under the dash. Its on the heater housing. This is what actually controls the electric motor and it changes your speeds from high low and two med options. The switch could be out also its not unheard of. But more likely its that resistor