The backup camera is mounted in the trunk lid, so it's the same temperature as the trunk is - warm but not "hot" and not exposed to the weather...I would prefer to avoid the back – people above reported heat issues and any potential water issues from up under the bumper. This is why I would put everything up near the dash. But good explanation!
It might also be cool to add another camera - front looking which would also be easy to do...
My preference for plug-in solutions like the one I described is that I like to be able to restore the factory wiring with a minimum of fuss, and the pass-through makes that easy. The other reason I prefer this approach is that you can build all the electronics and wire harness components on the bench then install them - there's a very limited amount of work to be done inside the car. I'm a big fan of "easy" and this approach makes installing this stuff very easy and a lot less error-prone.
As for heat and water issues, you can rely on Ford's testing during development because the connectors you get from your favorite electronics supply company are the same manufacturer and part number as the connector that came from the factory. Not just similar, but literally the same. The subcontractors in Ontario or Michigan that make the factory wiring harnesses buy their parts from the same suppliers you're buying from. If the connector in the car has a weather seal, pin locks, a retaining latch, a 105C thermal rating and a Christmas-tree style panel retainer to hold it in place, then you can buy a connector for your pass-through harness that has all of those features too.
As for a second camera, the place to wire that in is up in the center console behind the FCIM. You can buy aftermarket harnesses that just clip in and make it work.