I know this is an old thread but I am having a similar problem. I installed a C-123 in my 07 2.5i with the aux and a pig tail for it. I grounded wire number 4. I tried it out and everything worked great! After I put my dash back together, AUX music only plays on the right side of the car. CD and radio play on all speakers. I have no clue why. The aux cable is brand new just bought today. Right when i realized it, I wiggled the AUX cable a bit and it worked on all speakers and than it stopped and only worked on the right side again. Any help would be greatly appreciated!