Can you be persuaded to forgo FM for a gadget that fools your car stereo into thinking it's a CD changer so you can get a better quality signal? I spent months with a reasonably popular FM transmitter and it gave me nothing but frustration...never again. Many factory car radios are supported by these gadgets. You'll end up spending about $100 plus installation if you aren't up for putting it in yourself.
Check out Dension, Dice/AudioVox, PAC Audio, Grom, Blitzsafe. Christian at
Enfig is a good resource if you end up buying from him.