The problem is that the XM and Phatnoise both want the same 3 pins (18-20) in the large 20 pin connector on the back of the radio (talking 2004 factory XM here)

Remove sat wires from those pins (Preinstalled by Audi) and note what position they were in. Install Phatnoise cable in the 20 pin connector already in the car per instructions with it (cable), EXCEPT those 3 holes. Leave them empty for now.

Obtain 3 new connectors from the dealer and install in those 3 holes. I'm sorry I don't have the part #'s at the moment. Pin 18 is ground and BOTH the Phatnoise pin 18 wire and the sat pin 18 get spliced together to the new connector in pin 18.

Pin 19-20 are signal. Connect a 10kohm resister to each of these (4 total) on the phat harness and the XM harness. Now connect both pin 19's to the new pin 19 and both pin 20;s to the new pin 20. Resister value is approx, if you cant find a 10kohm, anything within 5% or so will be ok.

ie:

Pin 19 XM ---> 10k resistor \_______ Pin 19 on radio
Pin 19 Phat --> 10k resistor/


Do the same for pin 20. Clean up and insulate your work. I recommend heat shrink tubing.

Enjoy.

Unfortunately there are no plugs available from Audi to do this automatically, I'm working on making a prebuilt harness if I can get the plugs.

Note that this creates a passive audio MIXER, not a splitter or switcher. If you are an audiophile, it will cause about 6db signal attenuation, if not an audiophile, you probably don't know what that means and wont notice or care. If this is not acceptable, just rig up a switch to switch pins 19 and 20. I prefer the first method because its "invisible" and doesn't require anything but pushing the sat or cd button on my radio.

Let me know how it works for you, and if you need any more info. If you share this info with anyone else, please give credit to me for experimenting/discovering it.