This is the slickest thing I have seen. The I.C.E. Link+ works seemlessly with your car's factory or aftermarket stereo system. It is available for a limited number of OEM car radios and a few after market ones. Not inexpensive, but few good things in life are.

http://support.dension.com/dension/icelink_info.htm

Another option is the monster product - the iCruze

http://www.monstercable.com/icruze/

Good Luck!