I just pulled the factory rear wiper & washer buttons. I have a 1997 D90 NAS.
Aside from the lights+ground, each button only has 2 wires connected.
The Wiper button has one (green) wire and one (green+red) wire.
The Washer button has one (green) wire and one (black+green) wire.
Looking at the Carling wiring diagram from Jason... I am missing one brown+green wire (wiper park) from the factory harness. Did they remove that from certain model year?