The factory head unit has two channels...LEFT and RIGHT. Both those channels are powered, and go to the factory FOUR CHANNEL amps front and rear inputs by splitting the wires into two as they travel to the amp. The amp then powers the signals further, and send them on to the front and rear speakers.

I have an aftermarket head unit with RCA FRONT LEFT and FRONT RIGHT pre-amp out's going to the factory amp. When I get the time, I will run RCA's from the REAR LEFT and REAR RIGHTS on the head unit to the rear inputs on the amp that are now being fed by front signals.

1. The factory amp is under the 45 degree sloping section of the passenger side foot area that runs up to the firewall. This is why the passenger side has less leg room than the drivers side.

2. Yes it is possible, but I think pre-amp outs to the factory amp will probably get you a better sound than the head unit's tiny amps. This is double useful if your head unit has an option of turning off its amp entirely...improves sound quality, and reduces heat on internal dash components and CD's, etc. that you stick in the head unit.

3. Yes, you can just stick an RCA connector on the + and - wires running to each factory amp input and plug those into the back of your head unit. Works beautifully and gives you a much more relaxed and noise-free sound than the stock head units powered outputs going to the factory amp.