never heared of a dash mounted cd changer! nice, the only advice i can give is dont go back to the 'professional' audio guy again. i have no clue about vw audio upgrade parts but from my general knowledge ANY manufacturer audio upgrade is designed to cause problems, they want it to be awkward to screw any money out of you, they make their dollar off cars, actually more often on parts and servicing wheras aftermarket e.g sony, alpine... ect want it to be easy to keep your custom, spread the word and allow anyone to use their kit. generally anyway.
