Hmm..
Ik know an FM modulator isn't really that great but with the kit I got from a site in the uk (
http://www.veronica.co.uk/50mwsb.htm ) the sound was resonable.. I'm not saying I have wireless dolby 5.1 digital but it is acceptable and gives me less loss then highly comressed mp3's ;-)
the thing is most of these modulators go from 50Hz to 10KHz +/- 0.5dB which cuts of a lot of your bass and high tones leaving you with pretty bad sound. Count to that the too low transmission and you got ... well, basically, crap ;-)
This kit however goes from Response 30Hz to 15KHz +/- 0.5dB and gives acceptable audio quality.. I've been testing it over the weekend and it worked out fine for me with mp3's. this weekend ( as parts are comming in yippie

I'll see what it does with my Losless ripped WMA files ;-)
Eitherway, I had the transmitter in the back and my radio is a bad receiptor but still it was ok with the 50mWatt output.
It takes up between 9 and 16v so no convertors or power issues straight forward plugin and go. I went for the soldering war but if you are not too sure about that they have the 'done' one aswell..
As said, I know FM modulators are not THE solution but they are a solution to me not having the extra cash right now for the $350 new car radio WITH aux ;-)