FM Modulators are ok, but there are many adapters out there that allow you to change your factory CD changer control connection to an auxillary input so you can add an MP3 player or other device to your stock system. The sound quality is much better than FM modulation can achieve.
Check out:
http://www.pie.net – Precision Interface Electronics – cables/converters/adapters - check out the page. You want the following link which is on the right hand side about 2/3rds down the page. ANNOUNCING THE NEW PIEDIGITAL AUXILIARY INPUT CONVERTERS
and
http://www.soundgate.com/ - OEM interfaces and adapters
http://www.logjamelectronics.com/ - is an online retailer of both companies products.
Happy Hunting,Gadget