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I have a USB charger that plugs into the 12V dc outlet, your easiest bet may be to get one of those and remove the case then just wire it up using the connections to the terminals on it.
"Thats for playing music from a USB Flash Drive though isnt it?"
Yeah, those are cool. My daughter and her man in Germany have those in their VW and BMW. They carry their music on their key chain (with a quick release flash drive) and play it in either car they're driving.