Making a visualiser for my car stereo, need help.

I'm looking in making a visualiser for my car stereo, i would like to gather the audio information from either a RCA at the rear of the stereo head unit. Any help and guidance on this, in particular gathering the audio information from the RCA and putting it into the Arduino for it to read and display onto the visualiser circuit (I'll be using this one, a bi colour LED dot matrix Adafruit Bicolor LED Square Pixel Matrix with I2C Backpack : ID 902 : $15.95 : Adafruit Industries, Unique & fun DIY electronics and kits) I'm also using an Arduino Uno.

I suggest you look in the playground for Arduino frequency analyzer projects, because there have been similar projects in the past.