There are three methods that the wiper motor gets power.
The first, obviously, is when you raise the stalk to position 1 or 2.
The 2nd is when you pull back on the stalk, to the momentary position, which also energizes the washer pump.
The third is from the park switch, which is located inside the wiper motor.
If the wiper is any position other than the bottom of it's stroke, the park switch contact is made, and will continue to be made until it goes full round to the end of it's stroke.
If your motor continues to run more than one cycle after you return the stalk to the off position, it is because one of those three circuits is still made. The stalk switch may be off, but a bad delay relay is screwed up (that's a technical term used to describe an unknown defect) The stalk switches are pretty flimsy, but the contacts rarely burn or fuse because the switching current is handled by the delay relay.
To check the park circuit switch, you need to remove the lower dash cover, then remove the four screws holding the tin cover onto the wiper gearbox.