How does the "premium audio" external amp work?

[mhardgrove]

I have a 09 Versa and purchased a PY04F unit from a 2010 Sentra with the Rockford Fosgate Premium audio package. I did not know at the time of purchase that this system was externally amplified, and now I need to figure out a way to install an amplifier, and for now all I want is to be able to use the 4 factory speakers. After accessing the self diagnostic I confirmed there is no internal amplifier to power the speakers. Where I am confused is when it came to hooking up the premium radio, I noticed that my Versa has 2 sets of plug harnesses, one for the antenna, and one for the power/speaker wires. Both harnesses hooked into the "premium radio" just fine and the unit works, just without any sound coming from it. The speaker wires hooking into the head unit is where I am stumped, as I am wondering how the amplified signal goes from the amp back to the speakers.

I know that I need an amplifier, and there is a harness plug on the rear of the radio for a wire harness to go from the head unit to the amplifier which also has a harness plug rather than rca jacks, but can't find information on just how the factory setup works exactly. Where the power/speaker wires plug into the back of the radio, how does the stock amplifier work? Does it feed the radio signal from the head unit to the amplifier, then does the amplified signal come back to the head unit, then the amplified signal through the radio harness for the speakers? Viewing photos from what the amplifier looks like it appears to have various plugs coming from it, no RCA jacks or speaker wires, which is why I am confused. I want to believe the amp sends the signal back to the head unit then through the radio to amp harness, but cant find any information supporting my belief, nor any photos or wiring diagrams.

If there is an aftermarket plug that converts the radio to amplifier wire to an RCA jack setup I may install an aftermarket amplifier and just run wires independently to all the speakers, unhooking the factory speaker harness entirely. I have installed many aftermarket radios with rca jacks in the rear like this in the past, can the Nissan radio be converted like this?