[Heybuck]

Don't forget, the wiper motor works by when you turn on the dash switch, you are actually completing the grounding of the motor.
By that I mean, the wiper motor has continual power to it and when you operate the switch, you are actually completing (or breaking) the circuit using the ground wire.

So if it works on the bench, I'm thinking you are seeing it park properly when you remove the ground wire. If that's so, I think its a matter of adjustment in the motor so when you break the ground wire, the motor continues to operate till it cycles round to the point where the wipers are in the park position.

Bad explanation I know.