I just completed this identical installation. I had a wiring diagram for the factory bu camera and tried to locate the power supply for that without success.
I ran a separate wire from the backup lamp, neatly along the gasket, and then alongside the flexible boot which goes into the lift gate (attach it with a lightly tightened cable tie. This will get power to the transmitter.
I connected a second ground wire to one of the bolts holding the wiper motor, and routed down into the gate. I secured the transmitter to a brace with 3m double sided mounting tape and a cable tie. Gather all neatly with cable ties and secure them.
I led the camera wire through the license plate light, by removing the assembly from the outside, routing the cable ( leave enough slack for removal ), and reinstalling the assembly.
Test the installation before you button it up. My works great. Hope this helps.