If you can, avoid the FM modulator...the sound is SOOO much better with a direct connection. Although I have an aftermarket head unit (with an extra jack run), you might be able to get an aux out (RCA jack) run out to plug into the default stereo.
If that's not an option, I've had good experience with iTrip. However, if you are in a urban area, it might be tough to find an open FM channel.