Dear Jason,
We have same Webasto heater installed Nov 2006. See one pic attached of the front, same as yours.
Best pic I could take of the rear shows wiring. I have never had to trace it back to the heater unit, but, like yours works perfectly. Yes, the toggle switch can choose cold air from outside or hot air which feeds 2 cabins, the saloon and the shower/toilet.
If yours is working without the yellow and brown connected suggests to me that maybe it was connected to something else? I would insulate the ends at this stage to avoid them making contact with any other wiring.
Regards
Chris