Early this morning, after a night of freezing rain, i noticed the rear wipers running even though the car/ignition/wiper switch were all off. truck had not been started in over 24 hours. was unable to stop motor from running except only by disconnecting battery lead terminal. on trip to local mechanic hours later, motor would not stop, except when I turned-on front wipers – then the rear motor would immediately stop at whatever position wiper blade was in. motor would not park blade. only tripping the front (separate) wiper switch would interrupt power to rear wiper motor. At mechanic, problem would not reappear. learned later that similarly, the front wipers would not quit two weeks earlier. at that time, the front wipers eventually stopped some time after ignition was killed and vehicle sat idle with all switches off. Common thread is problem appears during rain storms and does not manifest during dry times.
I suspect that the wiper motor relay is bad. I had the same problem on a 2002 Dakota and it fixed the problem. The relay is not sealed very well and is very susceptible to intermittent failure when it’s humid outside.
Failure to stop is often caused by the park circuit in the motor. The wiper motor is always ON. When the switch is turned to OFF, power is cut at the parked position and the motor stops turning when park is reached. But a wiring diagram would be necessary to be certain how that function is controlled.Also, some vehicles have automatic wipers that are turned on by a wet sensor.
@ missileman and @ Rod Knox. thank you both for your advice. I will explore both solutions.