I've done a few minutes of research on retrofitting the NAV screen into the Z4. Obviously it's doable, with the scariest part being the fact that you have to cut a huge hole in your dash.
Right now though the one thing that's holding me back from this project is the cost. Looks like it's about $700-$800 in parts. Thing is, I don't really want the screen for the NAV, though it would be nice. The thing I really want it for is the radio display. After 5 years of Sirius tuners with all the artist/song info, I find the standard radio display painful. So my question is this...do I need all the parts of the full NAV retrofit installed to get the radio display working? Or can I hook it up to the radio, then add the MK IV NAV controller later on down the road? I'm guessing not, but figured it was worth asking.