Jumping front speakers off backs with amp?

Ok guys, I'm new to wiring my own stuff up, I've done a lot of mechanical stuff but never really messed with wiring. I have two specific questions though I want to amp my speakers and do so without pulling my doors all apart, I was going to wire the (+) on the front/rear together at my head unit as well as the (-), then just run the amp to my rears. The thing is, the wires off my head unit were spliced into the harness for the speakers. So do I wire the front/rear off the head, then front/rear in the harness, then wire them together?