I hook mine up to the Monster fm transmitter (using a Universal http://www.cellphoneshop.net/un2to3hespad.html2.5mm to 3.5mm Headset Adapter)

It plays through one of 8 fm preset channels.

It also works for hands free (as long as you dont mind the chance that there may be another person listening to the same channel that might pick up your phone call, tough they'd only hear half of the converstion)

or if your car has the input jack for an mp3 player then you could plug your cell phone into that jack with the same Universal http://www.cellphoneshop.net/un2to3hespad.html2.5mm to 3.5mm Headset Adapter