Since you have the cassette in dash model there is another very simple but valid solution. I have been using this on my X5 for several months now with no problem what so ever and the sound is amazingly good. I just used a cassette adaptor and hid the wire behind the radio unit, down to the central column and up to the central armrest where my mp3 player sits. Tere is no visible wire at all. I have a pretty basic Rio Forge mp3 player and the sounds is already very good, comparable to what I get from the CD changer. I am planning to replace it by a better player soon but still plan to use the same system. I have tried 3 different cassette players before finding one which does not make any noise when the wheels rotate. Here is the thread I wrote on this:
http://www.xoutpost.com/x5-e53-forum/...assette+player