These are garbage pictures so I'm not sure how much they will help but for my interior build I had to extend the tail light harness, while I was in there I ran new wires back for a trailer 7 way. I went through the floor in the back that had an existing grommet. you can then tie everything back to the crossmembers and frame out to the bumper.
(I think the red circle is roughly where that grommet was but I had already covered it up by then.