2001 Toyota Solara Aux In with jbl

I am trying to make my own aux input for my 2001 toyota solara. I have wired together 8 relays that when switched on will disconect the signal coming for the radio an revert to whatever i plug mymini jack into. My problem is that i am able to find the positive (+) line level speaker wire but am unable to find the negative line level wires. I think there is a common ground but am unaware of how i should go about wireing them.... if you are still confused try and figure out how you would wire in an amplifier to this system with the low level inputs on the amplifier and tell me what wires you are using. ...If you could point me in the direction of a detailed wire diagram i may be able to figure it out...thanks

I know I could do an fm mod but i was it to be a direct signal. and its kingd of a prject i wanted to do for fun...thanks for any help