For those looking to install a backup / reverse camera, the Navtool module is currently on sale until Feb 16th for $200 (half price).
https://www.navtool.com/bmw_Z4_2003-2008.aspx
You'll still need to pick up an actual camera, but this module claims to be plug and play if you have factory nav. I just ordered one, so I will post a review when I eventually receive and install it. Now it's time to hunt for the camera!