Something sort of like this? This only has one plug, but you could buy the other plug from BMW and then just splice the two together to make the plug that you need.
http://cgi.ebay.com/BHA9003-VW-BMW-0...QQcmdZViewItem

Another angle you could take is to snap the pins out of the existing plug in the car's wiring. Then, buy replacement pins with wire pigtails from BMW and snap them into the plug. Then you would have both sides of the audio wires to splice into your amp inputs and outputs.

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