In my experience, FM transmitters suck.
For my SUV (Chevy Suburban), I found an interface device that
connected between the cassette player and the radio head unit.
It allows me to tap directly in to the audio connection. My stereo thinks
I'm playing the cassette player when I'm actually using my iPod.
The sound is perfect and there is no noise or interference that you
get with the FM transmitter.
See this:
http://pac-audio.com/productDetails....&CategoryID=24
In this particular case, the device actually has two inputs so I could
also tie a DVD player into my car stereo.
Another place to look is here. I tried one of these but it had some
alternator whine in the signal:
http://www.pie.net/