Make sure who ever you go with, have them directly connect it to your system. DO NOT get it installed via FM Modulator, especially if you intend to play MP3's. Its sounds like crap through the modulator. I'm still trying get the installer to wire it in direct. Save some headache and get it right the first time (if you intend to get it installed via 3rd party)
And don't let them tell you it can't be done. llajumpvid had a great thread on exactly how to do it.
llajumpvid can you repost link in this post?
Peace.