Trailer or motorhome? We can access many drawings for motorhome and might locate it on drawings, not so much on trailers.
Another point to check might be if you have the original paperwork as there should be a price list showing what options were on the unit.
Example here if for motorhome:
As an alternater, assuming it is MH, here is the diagram showing where wirs were installed. Can you locate this switch on the drawing?
Page one is left/drivers side and page 2 right side. If you can locate the switch, being more familiar with the layout you can tell us where on the drawing and perhaps we can sort out where the wire comes/goes and the purpose?
Best of luck! Just consider it a good mystery to try to figure! Who did it and where.
Is this the right rear where a wardrobe is located? Checked for a light in wardrobe?