http://www.iphonefmtransmitter.com/
Although the new iPhone 6 cannot connected directly to a car stereo, using an FM car transmitter allows any car stereo with a radio to play music from an iPhone 6. This is an ideal solution for older cars, which usually have basic stereos with just CD player, giving limited music choices.
Switch to the radio on the car stereo and scan through the frequencies until you find a frequency that is not used by a commercial radio station or is only receiving a very weak signal. The best frequencies are ones that also have clear frequencies below and above them for better sound quality.
Plug the FM car transmitter in to the iPhone 6 headphone connector. If the transmitter has a power supply lead, plug it in to the car's cigarette lighter socket.
Change the frequency on the FM car transmitter to match the 87.9 frequency found on the radio. Most car transmitters allow frequencies to be stored in as a preset so that they can be easily found in future.
Turn on the iPhone 6 and start playing stored or streaming music. Adjust the volume on the car stereo to a comfortable listening level.
Watch the demo video for more detail
