That’s easy! You’ve got basically two easy options to choose from:
1. The old school (yet effective) solution – cassette tape adapter for any audio device.
Do you still have a vintage cassette tape in your car? If so, the easiest and cheapest solution would be using the cassette tape adapters. You can use these cassettes with almost every standard audio input (iPhones included, any iPhone will do).
Just insert the cassette into the car’s audio system, plug the audio cable to your iPhone and press play.
2. The better solution – the FM transmitter
All you have to do is search for a vacant FM station on the radio. You can recognize it by its white noise. Then, plug your iPhone to the FM transmitter and select the radio’s station number. Press play. That’s it!
This one works well for all the latest iPhone models including iPhone 4, 4s and 5.