LOC install and amp bypass GM

I am trying to install an aftermarket sub and amp into a 2002 GMC denali. I have the aftermarket amp hooked up correctly and wired to the sub. I need to wire into the existing speaker wiring with my line output converter in order to have RCA outs for the amp/sub. I can't for the life of me find where I can wire this in and make it work. I understand that I have a factory amp that I have to bypass. I don't get how: A. I can't just wire into the existing speaker wires before they make it to the amp. and B. What are the colors for these wires? I get wiring stuff from the internet and they only seem to be correct about the speaker wires going from the factory amp. Shouldn't there be similar inputs comning from the head unit to the amp? Should there not be (+) and (-) for each speaker being fed into the factory amp? I've eatched about a million youtube4 videos on this and they talk about building a harness to bypass this amp, but that makes no sense to me. I don't care if the factory amp is pushing sound to the speakers that I'm not hooking my LOC into. (unless that's going to sound bad for some reason). There are others that show people bypassing without doing this, but I can't seem to find the correct wires and I think it's becasue the denali has a total of 11 factory speakers and most of these videos are on tahoes or silverados. So far,the only way I've been able to get sound out of my speakers is by hooking into the rear door speaker wires that are coming OUT of the factory amp. But, this sound is very low and doesn't hit hard. For the record, I have hooked up an aftermarket radio with RCA outs and got the sub to hit like it's supposed to, so I know all that is hooked up correctly. Also, I am maintaining my factory head unit in this vehicle. Anybody that can help, please do. I have literally been working on this for 2 weeks.