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Get a circuit tester prod from Halfords or Maplin - they cost about £2 or £3 and will show you which wires are live without you having to electrocute yourself.
I have been researching how to install a iPhone cradle which I also need power for. I was recently advised on here that it would be easier to pick up power from the cigarette lighter which should be accessible if uve got to the back of the radio. I'll be trying that this weekend.