I am a brand new Audi A6 owner havinh just purchased a used 2007 A6. While it has the NAV buttons, it apparently doesn't have the control box. Is this something I can add? If so, besides the dealer who will likely charge me an arm and a leg, where do I start. Thanks for any input. P.S. Also, this may seem like a dumb question, but what is the difference between MMI & Nav+?