If you want to use your smartphone, iPod, MP3 player or tablet to play music in your car without a head unit, try one of the solutions below.
How to Pair an iPod | Car Audio
You can basically make your device function as a main unit by using:
To use this method you will need:
There is no easy way to bypass the head unit, connect a device directly to the car speakers, and then have it work the way you want.