From what I remember the 3 speed wiper motor is a shunt type motor as thats how they change speeds by grounding in a shunt field winding to slow the speed. On that type the switch does the grounding but I cant be positive without seeing the wire diagrahm of that wiper motor. The wiper motor on some got fed 12 volts at all times and when you turned them on the switch would ground the series or/and the shunt windings and when you turn the switch off the park switch kept it grounded until the park switch contacts open as the wipers are on the bottom of the windshield. I honestly have not had to troubleshoot that system for many years as I have not had any problems with mine or my friends cars. But its a good chance its the switch and of course either the switch or motor can really cause the problem. Also most Mopar wiper motors have circuit breakers in the motors and/or the switch. Ron