2007 Astra - accessing wiring behind radio

I have recently replaced my handsfree kit with a new one of the same type as the old (Mr. Handsfree Car Kit Genius) - I like the kit but the old one, which I have had for a while, unfortunately broke mechanically.
The new one has come with the hard wiring kit thrown in. In order to use that, which I would prefer to do, I need to get at some wiring behind the radio, or in some other convenient location.
Can someone advise me how I get at the wiring behind the radio or in another convenient location, and also which wires to use? I need a positive wire, preferably that is only so when the steering is unlocked, an earth wire and if possible a wire for the radio mute signal.
Phil Reynolds
mail: snipped-for-privacy@tinsleyviaduct.com (though I'd prefer followups on group)
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