Powered mine from the wiper motor, used a meter to test for the ignition on wire, forgot the colour was a while ago I fitted one.
I run the cable through the wiring gaiter by the left hand hatch hinge, tucked under the headlong, down the A post, above the glovebox and through large whole for cables behind radio. Earth wire connects to the earth stud on right hand side of hatch.
My head unit switches the camera oon when going into reverse gear automatically, hope yours does the same.
I was looking for a rear camera retrofit, but all "OEM" kits available from websites like Kuffatec and Advanced-InCar are criminally expensive (seriously, 350-850 pounds for a camera that's worth not even more than 50 pounds!!)
I've found some aftermarket kits on eBay, like this one, and I'm pretty sure that they should be compatible, although I'm not sure if it will work with the RNS-E, perhaps with the custom firmware, see section 8.1.5 in the changelog here.
Which kit do you have? Is there a schematical guide available?
Unfortunately, ETKA doesn't list anything about a rearview camera, so I got no luck with that.