mike, although you don't have the cd changer, if you have the wiring going to the back of the car for the cd changer, there are so many options. checkout posts here for mp3, ipod connectors. one unit i remember is GROM, there were so many others.

in fact not just a 3.5mm aux you can go for a USB interface as well (so that you can just copy music into a USB stick and stick it in the boot with thousands of songs to select from, literally). if you're not so adventurous you can just fix a 3.5mm aux.

there are positives and negatives (some love it it, others complain of bad sound quality).

in any case its worth a check on those if you have the harness for the CD changer already in (most probably you have it, cos how BMW configures cars is they actually build all those in, and just don't fix the unit if its opted out)

i almost threw out my cd changer and fixed one of those; eventually i thought 'naah, i'll let the cd changer be and just fix an aux cable, as my car supported it)