Not sure on the voltage but you should get some radio even without power throught the blue cable. The rear screen is the powered aerial. Have you tried swapping the beige/white connectors over? Also try just one from the adaptor into the white then with this into the beige. Then try the other from the adaptor into the beige and then the white. On each of these you can connect the blue wire and disconnect. I did this with the stereo out of the slot rested on the front seat, the cables from the car managed to reach just!
