Affraid i didn't find anything either, and wanted to do the same thing.
I resorted to buying the cable from expansis and "Adapting it my self"
The project was fairly simple, and if your happy with a soldering iron and a multimeter you should be fine.
I now have a small connector on my dash that will disapears under the dashboard to a breakout box. All i basicly did was chop off the box that has the two female connectors in it, extend the wire with an old cut up parralel printer cable (multicore and was lying arround) and then terminated in a project box from maplins.
This has two headphone sockets, (audio out stero, and mic in) as well as a USB socket to charge the phone. (the plan was to have the USB socket be usable for data... but that bit went a bit wrong...
Still from the testing i have done, i now have tomtom and WindowsMedia mp3 play back comming through my car stero, and my VarioII reports to be charging. Havn't had a chance to test it on a long drive yet, i think i might still need a beefier 12Volt to USB adapter.
If anyone want more info on what i did, i'd be glad to help.
here are some pics, sorry v bad Q....