Just to jump on the bandwagon here, albeit a bit late to this thread, BMW is free to not pre-wire the cars that they build as long as a viable alternative exists; which it doesn't!
My SA strongly advised against installing ANYTHING aftermarket, especially anything electrical, as you will most likely void a very large portion of your warranty because almost everything is tied into the electronics in one way or another. I just replaced the battery in my 2006 E60 550i and had to pay for an hour of coding!
I'm not the litigous sort, but at this point I believe that there is a strong case for a class action lawsuit. I've never owned a car where I can't touch the electrical system for fearing of 1) voiding the arranty; 2) having computer errors when you change anything - you run this risk just changing a light bulb!