A common fault with the 500. The wiring to the tailgate, where it passes across the hinge inside a rubber sleeve, flexes and chafes, eventually wires either rub through their insulation and touch together, or wires break. Your issue could be breakage, where current is trying to find an earth, but more likely, the power feed is touching the park feed.
The wiper motor has a permanent power feed to the park circuit, which allows it to park when turned off. Then there is a switched feed. if these two touch, the permanent power goes into the normal feed, and the motor runs continuously.
It could be an internal fault in the motor, but this is uncommon.
There are repair cable sets on Ebay, but need some skill to cut and connect. If you, or a family member are handy with such stuff, they can fix it. If not, you'll need a garage to do it, which will cost a fair bit of labour.