You can use any 3.5mm to 3.5mm cable. There is nothing special with the BMW one.
You can find it at Radio Shack or any computer store.

I haven't tried a phone yet (using iPhone Y-Cable) but I used my iPod a while ago and the quality was fine as far as I remember.

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2011 ///X5 M (On Order)

2009 X5 4.8 M Sport Space Gray/Black Fully Loaded

2007 GMC Yukon XL Denali Loaded and Customised