Or you can trace the reverse lights wires back to where they live in the body. But they might come from the roof.
All wiring into the liftgate comes from above the headliner - notice that's the only location with flex-duct between the body and the liftgate.
IMO the best source for finding this sort of thing is
the ~$25 Ford WSM subscription. Unlike the free alternatives, with the fully indexed and interactive WSM you simply look at the backup light wiring diagram, see the connectors along the full length of the backup light circuit, click on the hyperlink for any connector before the light itself, and you can then chose to view it's location in the chassis and the pin-assignment/wire colors at that connector. Easy-peasy thanks to hyperlinking in the Genuine Ford WSM.
Bob's yer Uncle - tap what you need at a location more convenient than in the liftgate and avoid the hassle of snaking your new wire through the crowded flex-duct which (IMO) is a PITA.
Trust me, you'll waste more than $25 in aggravation figuring out a solution without the WSM.