Looking at the date of registration I'm assuming you've got an e60.
I hope the following will be of some use to you - on the e39 each and every car to leave the bmw factory was installed with the wiring and bracket to install a cd changer in the left pull down section of boot lining. All you have to do is find a changer - ebay or I have seen clones from big name companies for approx. £150 - and slide it onto the bracket then plug in the prefitted harness, hey presto your stereo head unit recognises it and you're away.
As I said this may have changed on the e60 but if not, fitting a unit is childs play, it's just that BMW like to charge through the nose for everything.
If I get a chance I will ask a friend of mine today and post an update.