Thanks for Frau... :^)

They explained exactly what you were trying to: some resistance thingy. Actually, the switch is not used at all. They ran a new wire and connected it to the wires they removed from the switch. No wires are connected to the switch, which is kind of sad...

I got the scratching sound too. It was related to dried front speakers on my car. Changing the speakers resolved it.

Hoping this helps.

Nicolas