I just wanted to thank to OP and BeeQuu for the tips. I have a 2004 C32 AMG and I too had the white harness plug.Just some additional info here.
Enclosed find the drawing of the wires on the backside of the radio unit.
The white plug mentioned in several post is X39/36.
I have just installed my Aux and made a female mini jack input in the cupholder on the upper right side. I have only used 3 wires, Brown (BN) for the Ground and Black (BK) and Yellow (YE) for the right and left + input.
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I took the tip from BeeQuu and made my own cable using a 3.5mm audio plug by cutting off one end, attaching female pins on the left, right, and ground wires. Then I heatshrinked some plastic tubing over them and attached them to the pinout of the existing white plug (per BeeQuu's diagram: 1-ground, 2-left speaker, 3-right speaker). However if you don't care about keeping the harness intact, it would certainly be easier to just cut off the harness and hardwire directly to the existing wires but be sure to note the pinout as my cables were not the colors mentioned in the wire diagram. If you look closely at the white plug you'll see raised numbers on the plug (1, 2, 3, & 4). BUT if this is super alien to you and you are not familiar with electrical I would just shell out the money for the harness. <i>This is for those who have modded before and want to save $40.</i>
I was able to take the 3.5mm audio cable and place it under the carpet of the passenger seat and pull the plug up into the center console cupholder cubby so I can charge the iPod with the 12v plug while I am playing it.
I am purchasing a bluetooth receiver (something like this: Amazon.com: Sunnest Wireless Bluetooth 4.1 Audio Music Receiver Adapter with 3.5 mm Stereo Output and Hands Free Call for Most Smartphones, Tablets, Audio Player, Car or Home Audio Music Streaming Sound System.: Cell Phones & [email protected]@[email protected]@http://ecx.images-amazon.com/images/I/[email protected]@[email protected]@41jSYdkUbsL) to use with the 3.5mm plug for streaming from my phone. I could have just put the cable and receiver in the glove box, but again, I want to be able to access the 12V easily for charging the receiver as well. I'm so happy to not have to burn CDs anymore. THANKS!