Thought this may be helpful. Just ordered an aux input cable from Becker for my cdr220. Disconnected my CD input and plugged the new cable in.
Cost around $25.00 US.
Instructions were included and couldn't be easier. Cable has 2 rca outs and bought the monster rca to mini MP3 adapter.
Now play my zune thorough the factory stereo. FYI: Factory Stereo and amp are harman kardon - plenty of power so need to just replace the speakers next.
Part can be ordered by calling or online at:
http://www.beckerautosound.com/parts/index.html