Replacing Shaker 500 in my 2005 Mustang GT
 

Hey guys, so I decided to replace my stereo in the car, I notice there are 3 harneses on the back of the OEM radio, Futureshop gave me one harness, so before I took out the OEM i did all the wiring in my house, needless to say it does work, but should I be using another harness? The other harness has RCA cables hooked up which I believe is for the AUX input. I also have a switch on the drives center console kick panel thats also hooked up to RCA's? when i flip the switch it lights up but not sure what it does? Can anyone help me? Thanks.