I bought this brand new from Amazon, but I never installed it. My new car came with a Blue Tooth system - $25 picked up locally or $30 shipped.
Here's the description I copied from Amazon.
New Original BMW Auxiliary Audio Input Cable. You can connect virtually any audio devices - MP3 player, portable CD, cassette player, and more - to your CD/radio. The audio connection is mounted securely in the glovebox of your vehicle and has a 3.5 mm stereo connector. Audio from the auxiliary input is selected using the "mode" button on your radio.