I'm just checking you are aware of the VWCDPIC solution?
This does practically the same thing for most VAG (VW, Audi, Seat, Skoda, Bentley?) headunits.
Interesting, each VAG has subtle differences, and it has taken the developer a long time to tweak the code to work for all.
As of July this year, Ed is no longer making any units, so he may not object to "handing the batton over".
Here's more detail:
I have used the solution for almost 2 years, and it's very simple and effective.