Anyone Know how to combine the Rear Speaker signal into one full range signal

[Lightson357]

I replaced the Rear 6 X 9 speakers. But my signal (using a front Speaker signal Tap) has a lot of noise. Plus I am getting turn signals in the back as well. My Question is can I combine the rear speak signal back in to one signal and use that. Or is there a signal I can use before the Amp. Because that signal would be much cleaner with no noise. I will be replacing the front speakers as well. I am using a JL 300/4 as a amp to power the Rear and Front speakers. I think I tapped into the wrong signal. I have looked at the wire diagram and cant seem to find the signal from the radio to the amp.