Have a 2011 Legacy and on some mornings the wipers come on and you cannot turn them off. Took it the shop and they cannot duplicate. Turning car off and disconnecting battery did no good, soon as reconnected they come back on. Waiting for it to happen again so I can drive to dealer whlie it is happening!

there must be a sticking relay somewhere they could just swap out.

On my '03 there is a mechanical switch that tells the wiper controller if the wipers are 'parked', could be a problem with the switch or the wiring from the park switch to the controller.

 

Old cars were easier, they just shorted the motor to ground (switch was in the ground path) if the wipers were not in the 'park' position.

 

A schematic would help determine if it is likely a short or an open.