I have a 2006 X3 with NAV and HU as shown. I have taken the radio out so many times its become a simple process. I have never removed the NAV HU though but it looks simple. I set up a CarPC in the rear of the vehicle connecting it to an aux port cable I ran from the front HU to the left rear driver side compartment. In addition I have a Sirius OEM module and BMW OEM BT module installed below the NAV DVD unit. I also purchased an changer and kit for install but never have installed it because of the space it used in the center console. I was thinking of installing the changer in the E46 removing the one in there so I could play MP3's. The only thing is that my radio module wouldn't read the MP3 tags unless I upgraded it in the E46. You will also need (unless you already have it) is the rack mounted in the compartment to mount said additional equipment. I noticed there are a few types. One destined for the US and one Euro style. They are reasonable in cost.

Not sure how the cables are run from the front to the rear of the vehicle. I know they are not on the right side that I could tell when dismantling it so my guess is that they run on the drivers side but couldn't tell for sure.

Are you going to install the shark antenna. I believe that my Sirius, GPS and BMW Assist antennas are there. Interesting too that there is another cluster of wires nearby not connected to anything. One of the wires is an antenna cable and I wonder or tend to believe its for an additional cellular phone cradle connector?

Curious how much prewiring you already have in your X3? It would save you some time if some stuff was already there.

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E83 - Aero PKG Mods installed: Homelink mirror. CIS-IBUS video module, AUX, Rr Dr Release, stock wheels, Remote start, CarPC (Atom) & mud flaps - NAV - MKIV - NA Map DVD - 2011
E46 - 18.5 wheels/tires (summer), 17 stock (winter), Homelink, NAV software upgrade, BMW video module, Radio to V53, AUX, USB IBUS interface,BMW alarm install, CarPC(Epia), remote starter, tank battery & angel eyes. NAV - MKIII - NA Map CD - High Version 2008.1