Don't know how much help this will be but when I bought my E34, the boot (trunk?) CD changer didn't work. It turned out that the previous owner had managed to jam 2 CDs into the boot changer. I took the boot changer apart to remove the stuck CDs but even after removing the jammed CDs it didn't work properly, so I replaced the stero system (and partly because I've always prefered front CD players to boot changers). If you pull away the boot lining you might be able to see a connector where the boot chager connects to the cable from the head unit - maybe check the contacts of the connector to make sure they're not damp/corroded.