Thanks to ccfj1 for your help.
I have just called my dealer and quoted my chassis number to enquire whether my car is pre-wired or not. To my amazement, they have informed me that my car is not pre-wired. I thought all X5s from 2002 on were pre-wired.
Well, I guess I will have to abandon this project as the thought of ripping everything up inside the car to install a new cable loom is too much to contemplate.