I had the same problem with my 2003 RS6. The CD changer harness on mine has a Brown (ground) wire that went into a Brown plug that is now not used with the RNS-e. I cut it and grounded it to the chassis of the RNS-e and all is good.
OR simply ground the Phatbox itself to chassis and see if it works.
OR simply ground the Phatbox itself to chassis and see if it works.