If it simply for entertainment of back seat passengers, consider an AppleTV to save money.
You can output the sound to an adaptor. Try searching at Crutchfield for a solution. They have car specific adapators. Also, not sure if the Car2PC adaptor works with the new Jetta, but you can take a look at it.
If you want an install for the front seat as well, I'd still consider the AppleTV idea for the back and use the Mini up front as a server.