This is driving me nuts.
I have an iPhone 5 and a '09 135i. The car Bluetooth and the phone connects fine that way for phone.
I can find no combination of cables that permits correct production of music.
1) Original Y cable with 30 pin to Lightning adapter: Pandora plays fine, though I have to use the iPhone to change stations. The standard music does not play correctly; a single album will play with no way to select another. The radio only goes between songs on the album and the other playlists (and albums) when selected give "Empty List." When the iPhone is connected, it won't permit direct control of music.
2) Lightning to USB: No sound at all.
3) 3.5mm to 3.5mm: The sound produced by the iPhone is heard, but only the iPhone controls the music.