NavVideo has an iDrive video interrupt module -NV_BMW356 for up to MY2008, NV_BMW09 for MY2009-on- that can be used for back up camera.
http://www.navvideo.com/navvideo-models-nv_bmw356.asp
The NV_BMW09 module is not listed in the website, but it is available for $395 (no camera included).