Wire rear speakers from scratch?

[joejitsumd]

Okay, I could use some help and guidance from guys that know about car stereo/amp installs. I have a 2001 boxster with the door speakers. I swapped out the front dash speakers for Alpine speakers and I just changed out the head unit to a Kenwood BT755HD. The head unit on the back has 6 RCA outs (front, rear, SW), of which the front/rears RCAs are going into a 4 channel alpine amp in the front trunk (installed by the previous owner).

I have the rear storage compartment and I just installed two Kicker ks35 speakers on each side of the storage compartment, but I haven't done any wiring whatsoever to bring the rear speakers sound. I have Wayne's book which talks about using an aftermarket speaker kit and plugging it directly into the head unit, but I am not using any special connectors and I no longer have the factory head unit. I notice when I use my fader that the door speakers seem to be wired to the rear channel. What I want to know is how do I wire these rear speakers so they can be involved in the system? I'm not expecting dramatic improvement but I have already bought the speakers and want to use them. Can I wire the rear speakers directly to the rear speaker wires on the kenwood radio harness? (Even though my RCA rear speaker lines already go to the amp). Do I have to find a way to splice the rear speakers into the outputs from my 4 channel amp? If so, are there any resources that could explain how to do this. This is my first venture into car audio so I'm reasonably green on the subject.