I just put a bandpass in the back of mine lol It houses 2 12" Kenwoods 1200W max (400RMS) behind plexiglass. Then bought a class D mono AMP 500RMS the wiring is cake because everything is in the back in the X5.
I just tapped into the Radio module wires and used a HIGH/LO converter grabbed the power for the remote wire to the converter from the module as well.
Everything is neatly tucked away including the amp as I fastened all the wires to the factory harnesses and then anchored the speaker wires along the well where the spare tire is. The speaker wires for the bandpass come out the black cap in the cargo bay and right into the bandpass.
Very clean install and very easy to do for the amount of money these places are charging you can get much more "bang" or "Boom" in this case for the buck DIY lol

__________________

2006 X5 4.4i