Can't say i'm keen on FM transmitters, they may have come on, but I often found them tricky to keep working whilst driving, particularly round London which has quite congested FM usage and you'd end up with a lot of interference.
A CHEAP way of doing things is the AUX input into the stereo in the boot. My initial take was a jack cable fed into the cabin via the ski hatch, but a Bluetooth transmitter in the boot using the AUX is a pretty good alternative, quicker/less destructive and you even have a cigarette jack above for power!
Eventually switched to a GROM-MST over the Audi/Denison MMI options. It's great when it works but the optical bus is a pain and sometimes it doesn't want to play, so despite better audio quality of FLAC i'd probably just go with the Audi thing as it just works!
http://forum.a8parts.co.uk/showthread.php?t=11114