Hello, Most likely you need a new control board for the motor. It is not the switch... I am new to GP's but not new to cars, so what I do not know is if the control board can be replaced without replacing the entire motor. If it can be replaced, check it or the motor itself first for cracks and or water that has gotten into the board. If no cracks present, the board could have just dies on its own.
For example on my Corvette, it can be replaced itself and if the windshiled cowl drains plug, water can build up and enter the control board, thus causing it to act weird like this. If taken apart and let dry out, it usually is ok afterwards.
What you are also experiencing is what happens when the HVAC blower motor resistors go bad. The default setting is high speed only, or the full 12 volts to the motor. It acts like this so at least the driver has a fan, even if full speed, in case of emergencies. I am guess here, but this motor too may default to high to at least give you wipers in emergency. They stop where ever because the PARK position, which is controlled from the control board, is bad.
Go to you parts store and ask if the board is sold separately or not, that will tell you. I would also check the electrical connection on the motor, if water got into it, it too can cause weird things to happen. This check is free of course, so start with the basics first, but I doubt there is an issue there.
