I just un-installed that exact system. I got a 2005 X5 and it had the kenwood ddx7015, nav system, and sirius tuner module. The wiring to make that system work in the car was ridiculous...AND, they left the factory headunit hooked up in the rear. I don't know if the people who installed it were idiots, but I spent 12 hours undoing the mess this weekend. They ran antenna, cd changer, speaker, and power wires to the back of the vehicle. They clipped about 30-45 wires in total (as in cut, not tapped). Then they made a mess of the harnesses. They cut the antenna cable. This was a PITA to reverse. I don't know if it can be done where it is easily reversible.

I decided to reinstall the OEM system because I didn't like that there was the factory headunit in the trunk, and I don't use Nav all that often.

If you have it done, take it to a reputable shop and ask them what they have to do to make it work, and let them know that you'd like to be able to revert back to OEM when you're done with the car.

Just my .02.

(Edit: the car originally had the factory NAV, so that's what I put back in)