The guy(s) at iPhone Music Transmitter sell a nice device that will make your life easier if you have some older cars that cannot connect via wire or wireless to your iPhone 5.
Wesly Wade sells a device that can be plugged into the iPhone and then transmits your music via FM radio. Once you set your car radio to 87.9 MHz on FM band you’re all set.
I can imagine that there could be some other nifty uses for this FM radio transmitter so hop on their site and pay up $49.99 for this nice gadget. Have fun!