Hi there. I have a 2005 x5 sport. I have the same dilema with my radio cassette. I have no use for the tv and it doesn't have the sat nav. (I kinda no where I'm going). Lol.
I just got an aux cable from eBay. It connects directly into a 3 pin connector on the radio loom. I use that for my iPhone ipod. Music mainly. I will get an aftermarket system later. But for the time being I'm over the moon with the cable. Very easy to fit. I'm in belfast.