easiest way to fins out it get a test meter or a 12 v lamp with some wires attached, and connect it to each wire in turn and the other end to the shell, switch the heated rear window on and see which wire comes live, then just to prove it, connect the other wire to the lamp (with the other wire now disconnected!) and turn the radio onoff and check that it too lighs up when you expect it to.

hth.