On a 2008+ E/M/T, the BLACK with GRAY STRIPE wire is ground (-), and the GREEN with BLUE STRIPE wire is +12V when the reverse lamps are on. Those wires are located in the harness of the socket connector of each reverse lamp bulb.
You can also find the GREEN with BLUE STRIPE (+) wire in the harness under the driver-side rear door scuff plate, if you don't want to run a wire all the way back to a reverse lamp bulb. Just make sure that it's the only wire of that color in the harness, and test it first. The other (-) connection can go to any metal part of the car.