Arduino:Car:Ultra Sonic Sensor

So I was wondering if anybody has worked with connecting an Arduino and a car’s audio system. We are working on a project that we want to use an ultra sonic sensor to activate a cars audio system on and off when their hand is at a certain projection. We have the working code, and can code threes features easily. But what I cannot understand how to do is wire the Arduino to the audio system of the car so that we can turn the audio off, on, and change volume. I heard the six is the best way but unsure how to accomplish this. Thanks for the help!

I heard the six is the best way

What is "the six"?

how to do is wire the Arduino to the audio system of the car so that we can turn the audio off, on, and change volume

Unless your car's audio system is designed to accept incoming data to do this you can't without hacking your audio system, which is an altogether other matter.

Maybe you could get the Arduino to "press" buttons on the audio system's controls that come out to external controls in the car. Best way to do this would be to use an opto isolator.