Do they stop at the same time as the ignition is turned off or do they park in the correct position?
If they stop when the ignition is turned off I would say the problem lies in the wiper motor. There is a switch in the park position if it has no power the wipers stop, but if it has power the wipers keep going. I think that this switch inside the motor housing has power all the time due to some fault.
The wiper motor has power to it all the time and only when this switch is opened the wipers stop. But if their is a ‘short’ before this switch which eliminates the switch the wipers will keep going.