Simple Audio-USB Adapter

[dandaman2]

Here's the deal.
A couple months ago I custom ordered a fancy new Mini Cooper S Convertible, which arrived in the US a couple days ago, and should be at my local dealer soon, if it isn't already. Among my options was a bluetooth system for my iPhone, which unfortunately was only available if I sacrificed a future-proof, high-quality audio-in for a USB interface for audio. I might get the stand-alone MINI Garmin GPS system, but I like keeping my options open, so I'm considering a TomTom dock instead, plus the TomTom/Navigon app.

The TomTom dock has mini USB for charging, plus an audio out. Will the mini USB handle audio out as well? If not, where can I get an adapter to turn the audio in to a usb in? I am willing/hoping to build it myself! Not an external soundcard because I can almost confirm that it will not work with a car.

I think this is the right place to put this because I'm looking for a piece of computer equipment, it's just that I'm going to use it with my car and phone.

Thanks!