this is E70 platform, so i am not so sure... but, from the early days of E32 and the rest, CD-prewire meant, that there were plugs in the back of the car: 3-pin for power/ibus connection and 6-pin/coax for audio connection... you get a CD changer (not a DVD), mount into the bracket (needed to buy bracket), plug in connectors, and voila!!! you got a CD changer...

I am sure E70 is the same way, just using the MOST instead of iBus...