As per my PM to you, this should work a treat but note that you will need a Fakra version and not the standard car aerial (connector) version in the link above (the picture above is correct though - or at least the aerial connectors are but the unit itself is not actually an FM modulator).
You could use a third-party unit like this one...
BMW AUX iPod Fakra Wired FM Modulator transmitter FMMOD4 iPhone MP3 Connects2 | eBay