Hi Andre,
It seems odd to me that all 2004 Bimmers were not prewired. It's not clear to me if you checked with the dealer to see if you could install Assist in you car. If they say you still can, then I'm 99% sure that your car is pre-wired. From what I know, the wiring is there but for verts, dealers say that you cannot have Bluetooth though they are happy to install Assist. What I did was buy the necessary parts and install it myself. I have used handsfree while driving 80 MPH with the top down, windows up, and the windscreen up and callers on the other end can hear me fine, though there is some background wind noise. I'll check with my local dealer and see what they say about pre-wiring on 2004 ZHPs.
fantom