I have more or less your problem with Bose Symphony II. If you want to play mp3, there are several options, but from my research I don't know if exchanging the Bose head unit is a good answer.
Possible solutions are casette or fm transmiter, and get a separete mp3 like a pocket pc. If you want better quality, you need an special cable (around $100) which will let you play sound directly through the cd port (meaning you lose your cd changer).
for more info check this site:
http://www.autotoys.com/x/cust/produ...9&cat=0&page=1