There are aftermarket head end units that have Bluetooth capability, and Leo recommends the Alpine stereos. There are a lot of FM transmitting devices, but in a city like Cincinnati, the radio dial is packed with stations. Leo doesn't recommend those unless he lived in an area with few stations.
Since he already has an auxiliary mini-jack on his head unit, he should get an A2DP Bluetooth dongle. Belkin has one called the Belkin F8Z492TTP Bluetooth Music Receiver for $25. The Chatroom says that the GoGroove also works great.