I recently bought the RCA/AUX cable and connected it to the port that was already installed in the car, but how do I make it actually work? Is there something specifically I need to do for the connection to work? I have an 08 Lancer

 

I have a 09 lancer sportback with the standard L + R Aux connector in the little cut out at the bottom of the console. To have the stereo switch to Aux press and hold the cd/radio button on the head unit. ( or it might be press and hold the radio button i do it without thinking.)