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You could try and run the cable in the existing convoluted tubing alongside the existing wiring. You would have to remove the various pieces of trim on the tailgate and inside the vehicle to gain access to either side of this point.
The easiest way however is to rethink your mounting point for the camera. I usually position them to the underside of the bumper and then find an existing grommet in the boot floor or make my own hole and install a rubber grommet for access.