Great idea! Sending it digital to the trunk, instead of analog, should tremendously reduce the interference.
Only thing is, if I'm reading this right, I think you might have your wiring mixed up. If I understand what you're trying to do, it should go standard male iPhone 30-male usb, female-female usb extension to the trunk.
Then you'd need a male to female 30 pin, because it looks like the Pure has a male iPhone connection, intended to dock with the phone. I'm sure you could find one somewhere. And then like you said, RCA to you amp and you're all set!
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2007 335i E92
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