You need to replace blower motor resistor.
replace the blower motor switch
Failed blower motor, resistor or switch.
If the blower high setting isn't working on your '97 Dodge Ram Van even after replacing the resistor block, the issue may lie with the blower motor itself or the blower relay. The intermittent operation of the medium setting suggests there could be a loose connection or a failing blower motor. Before replacing the relay, check for any loose wiring or connections and consider testing the blower motor to ensure it's functioning properly. If those components are in good condition, then replacing the blower relay could be a worthwhile step.
The blower motor speed resistor is burned out.
Blower resistor pack broken?
when a blower motor only works on high, it is usually due to faulty resistor board. Should be located in the duct near the blower. the purpose of this is so the air blown by the blower keeps the resistors cool.
Blower fan inoperative or only works on high speed.
replace the ignition switch
The blower motor resistor pack is defective. Replace it.
check coolant level
If you are saying that the blower motor only works on the high speed setting, then the Blower Motor Resistor is defective and needs replacing.