I did installed a back up camera in our '07 Navigator L.

Underneath the passenger's seat you'll find a green wire with a brown stripe. Actually, two of them!

One of them is the lead for the back up lights, and should you decide to use your camera strictly as a back-up cam, that wire is hot once you select reverse.

If you search the subject, or even posts by me, you'll find more details.

Good luck. For your intentions, the trailering harness sounds great.