Space in most boots is very tight. It may not be possible in some, so check it for extra space before going to the trouble of running your cable all the way up there.I may try this technique when I get my longer cable, as there should be enough length to allow a tidy fitting via the one of the cable boots into the tailgate wiring, assuming there's enough space in the boot..
After doing this in two cars, I wondered whether it would be easier to make a bracket of some kind that would attach to the headliner inside the hatch. The camera could mount to it, as long as the bracket would position the camera correctly. It would be nice to eliminate the hatch problem.