David, I have not got a clue as to what set-up might be for your wipers wiring but from a very dull memory, the park position was adjustable at one time by rotating the round tin top cap on the wiper motor once the screws were loosened off.
As best I can remember inside the top of the cap was insulated with a very thin sheet of plastic with a cut out in it a bit like a pie chart with circa 15% cut out and exposing the metal of the cap. There was a an electrical wiper arm mounted on the gear motor gear wheel that made contact with the top of the cap internally running round it`s circumference continually while the wipers were switched on. The idea being that the electrical wiper continued to provided an earth to the motor when the earth supplied by the wiper switch was switched OFF, thus the motor would continue to run till the electrical wiper hit the insulated section on the inside of the cap, at which stage the earth was lost and the motor stopped.
Adjusting the park position was achieved by loosening the screws around the wiper motor and rotating the cap. Some of this type of wiper motor had an earth wire connecting on to the centre of the cap externally.
Hope I have not served to muddy the water..?
