Rick,
The switch senses door status. When closed the switch completes a circuit that allows the inflatable gasket around the door to inflate if the engine in running and the transmission is in gear (not neutral).
Gene,
I checked with NW RV and BCS. Both had them in stock but were considerably more $ than the eBay version, which I had already ordered. If it fails I'll contact BCS or NW RV. NW RV part# is 16622291.
As close as I can tell, mine broke due to one of the plungers hanging up and not closing when the door was shut. In future I'll hit them with a bit of silicon spray every so often.
Steve