Hi Bjorn, welcome to the forum,
I've retrofitted BT to my 02 e46, mine also did not have any phone pre-wire, so i started from scratch and made my own retrofit harness, I used the radio modules quadlock connector to provide all that was needed for the 54 ULF connector (ie: power, switched power, GND, i-bus, tel-on, mute and so on), this is no different from a normal car phone kit.
This is what i came up with (not this exact one but similar, I made this one for a friend).
Your car is similar to the e46, so I see no reason why you can't do the same, after all the BMW ULF acts exactly the same way as any aftermarket BT car kit (nokia, sony and parrot etc), the only real difference is the I-bus/Tel-on connection.
Here is a link to what I did;
http://www.xoutpost.com/bluetooth/324...tml#post329623
If you have any more qusetions the just e-mail or PM me.
HTH
BR//john