Really? Very interesting. I've contacted my local car stereo shop to see if they can source the right Dension box for me. If not, it looks like I can order it from some German web sites for EUR 140 or so. (I'd rather buy it locally, if only that if/when it doesn't pan out, it's easier to return it.)

Footnote: Rereading my car stereo's manual, it appears that it supports MP3 CDs. At least I can burn something for road-trips until I get this Dension thing sorted out.