FM Transmitter ?

If your car has inputs or option for Bluetooth Stereo audio, use that.  It'll give you much better quality compared to FM modulation.  And you won't have to worry about nearby radio stations "running over" your transmitter signal.

If you go the Bluetooth route, make sure it's Bluetooth stereo music and not just headset/handsfree.

The required Bluetooth profile on the receiver is A2DP or Advanced Audio Distribution Profile.  Sometimes this is referred to as "Bluetooth Stereo" or "Bluetooth Music."

The Xenon will actually stream music over a Bluetooth headset/handsfree connection too, but the quality sucks.

In order from best to worst quality for music playback
Direct headphone
Bluetooth A2DP (stereo music)
FM transmission (with appropriate headphone adapter and FM transmission device)
Bluetooth Headset/Handsfree
Phone speaker