Den, it sounds like the cabin DC outlet was added by a prior owner. When I modify my electrical systems, I either markup existing schematic pages, or I create new pages. The other VERY useful thing I always do is mark each end of every wire w/ the function and w/ the connection point for the other end of the wire.
My guess is that the PO did none of the above.
If all that is true then a brute force technique may be required, to manually retrace the wire to the origin. Ensure that the wire is properly routed, avoiding pinch points and other hazards. I like to enclose all my wires in split loom for protection and support throughout the length, w/ wire ties and cushioned Adel clamps. Ensure that the circuit is properly protected w/ an appropriate fuse or circuit breaker.
