I just wanted to add that you should stay away from FM transmitters--they are garbage. The poster who mentioned an in-line one sounds interesting. I hadn't heard of those before, and that would probably solve the biggest problem of interference and a lack of adequate broadcast power.
That said, if you are going to get behind the dash to do something like that, why not just consider an aftermarket head unit? If you haven't shopped for one in awhile, you'd be suprised at how cheap they've gotten. Mp3-capable ones with direct front-facing USB ports are under 100 now (e.g., see
http://sector29.com/PRODUCT_PAGES/263/263-BOS735UA.html from a quick search). Many of those also have bonus features like HD Radio (if your service area provides it--mine does). I hear the DUAL brand line are exceptional for things like that (fairly cheap for bluetooth, usb in, hd radio). Something to think about.