Hi Robin,
That's exactly what I am doing.
But I have drawn out the wiring diagram from the supply via the switch to the motor (using your diagram) and I can't work out where the earth is coming from.
It's interesting that you mention the standard loom and a "wiper relay". I found this circuit
www.Vitessesteve.co.uk and have wired up my wipers the same way and now they works just fine..................
wiperdiagram by
MarlinBuild, on Flickr
It's a simple change over relay that isolates the wiper supply voltage from the park supply. It works, but for me, it still leaves the question Why??
But being pragmatic, it works, so forget it.............................!!