I am actually experiencing the same issue right now, #1 son is using the Jeep until he gets a new vehicle and he complained the blower only works on High so I had him pull the center bezel to give everything a visual inspection and we found the beginning of burnt plastic. I decided to replace the blower motor to prevent the typical high amperage draw from a tired motor. We swapped out the motor and resistor pack and everything worked fine except the 3rd speed. I had a spare Mopar fan speed switch so we swapped that out with the same results.
Willing to listen to any thoughts.