I have an 08 A4 and I'm about to replace my base stereo with a RNS-E upgrade kit. That RNS-E has already been programmed for my car and should be plug and play. After installing that I'm installing the factory CD changer that goes in the glove box. Is the CD changer plug and play or do I need to use a VAG COM to make it work if the car didn't have one from the beginning?