This sounds like you are going to need to replace the Blower Motor Resistor. The blower motor resistor is mounted to the lower right side of the heater-A/C housing in the passenger compartment, where it can be accessed for service.The blower motor resistor has multiple resistor wires, each of which will change the resistance in the blower motor ground path to change the blower motor speed. The blower motor switch directs the ground path through the correct resistor wire to obtain the selected blower motor speed. The blower motor resistor cannot be repaired and, if faulty or damaged, it must be replaced.
Disconnect wiring connectors from resistor.
Remove blower resistor mounting screws.
Pull blower motor resistor out of HVAC housing.