what is the device called that connects your iphone to your car stereo to play music?

If you are lucky, it's called an audio cable.

If not, a low-range FM transmitter. Just go to an electronics store and say what you did above.

To input audio from your iPhone only no controls simple to use you will use an

auxiliary audio input interface

http://www.oemautosound.com/c-20-auxiliary-input.a...

Or you can buy an iPod interface iPod integration,

This will show all the iPod iPhone information on the radio display, you can use Pandora livio radio or any internet music program it will also charge your iPod

http://www.oemautosound.com/c-19-car-ipod-interfac...

Stay away from FM transmitters they don't work on new vehicle for many reason the 2 most important are that the new transmitters power was cut down from 3 watts to 0.6wats by the FCC the second reason is because new vehicles don't have the antenna now is integrated in the rear window with a signal amplifier

good luck

Thats called an 'fm transmitter' nevertheless it comes developed-in to a telephones firmware, u cant down load application for it. But, u should buy transportable fm transmitters in an effort to do the detailed same factor. So do a local google seek for 'fm transmitter for ipod' and ull find one.

http://www.amazon.com/gp/search?ie=utf8&camp=1789&...

An "AUX" cable or an "AUXILIARY" cable.