Grrr - Mad Hat would appear to be correct as I am presuming your camera provides a standard video signal output, (hence the need for miniature coax cable - as suggested by MH), and the camera will then only require 12 volts and earth.
The most logical supply will be from one of the reversing light feeds, (I wouldn't expect your camera to be drawing too much current so it should not interfere with the lamp failure circuit - but stand to be corrected - as always), and you should find plenty of earth points close by!
You will then have a rear view camera that only operates when you are reversing - providing your reversing light switch, (XYZ), is operational of course.
Just for the record, however, the easiest route for cables, (from the front compartment back to the boot area), is via the edge of the vehicle, (simply pull off the inner plastic carpet/sill guards), and run your cables along there, (many of the vehicle cables are already situated in this natural valley - under the carpet).
Things get a tad more difficult in the rear compartment as you will need to pull the side cosmetic trim off and run the cable behind it.
I have only just reassembled my complete interior, seats and carpets - just about to start on the headlining.