The wiper motor is wired with a constant power condition that is keyed and fused from the main panel and the wipers are cyled when the motor recieves a ground potential from the dash switch and or park switch. If you turn off the dash switch when the wipers are not parked and the key remains in the power on condition, the park switch (located internal to the wiper motor module)remains closed to ground until the wipers park. Normally the wipers will fail to park due to a poor or missing ground at the motor. The park switch is providing the needed ground to continue the motion until the wipers park at home. The motor is grounded with an embedded contact at one of the mounting bolt locations. Many times this ground is lost during restoration due to painting or breaking it. Or the bolt loses it's ground to the cowl due to rust.
Hope this helps.