Remember (or take note) that not all X5 radios/stereos are the same. My radio is in the dash (as part of the Professional CD player), I still have the CD changer in the back but with no DSP I have no audio modules at all in the battery tray.
There's a lot of variation, so OrangeFurious wasn't being totally daft.
The battery tray can contain a radio/amp combo (the BM54 module?) or just the DSP amp module. Sometimes the radio or amp module is in the left rear compartment under the CD changer too.
If I was adding a Dynavin or an Eonon to my car, there's nothing to run to the back. All the wiring is up front, including all spearker wiring. I'd look at relocating the MID display to the boor/trunk though, so I don't lose the MID functions.
If your car has Factory Nav and/or DSP then you will likely need a loom from the front to the back, to connect the the speakers at the very least. Each aftermarket unit will be spec'ed for a specific factory config. so make sure you get the right one for your exact car.
acidgreeny, since your exact car has neither NAV or DSP but still has the radio/amp module in the rear, you'll need a loom run from front to back for the speakers, some signaling and the aerial.