Backup camera plug type?

Most likely what I will do, although I'd prefer to make an adapter for both ends vs. trying to solder inside the cap and headliner.

Just cut the connector off. One of the wires should have a shield so use it for your video. The others really don't matter. Just match it up with your camera and wire down which is which. Same for the other end at the monitor. After you solder them and cover them with some heat shrink, no one's going to see them.