this is the only app you need, provided your cars stereo has an aux connector. if not you're stuck with crappy fm transmitters.

Newegg.com - Scosche 3.5mm iPod Retractable Cable - Black IU3.5RC

 

if your car has a cassette player, you can get an cassette adapter that plugs right into the 3.5mm connector on your phone....easy and cheap. You can find them almost anywhere, walmart, eckerds etc.

 

Or look in your closets and see if you have a Sony Discman from the mid-90's when they used to ship with a cassette tape adapter. I saw one at CVS drugstore recently selling for $16 bucks... don't go there, way overpriced.