okay question: what would be the most feasible way to wire 2 door speakers and 2 rear speakers from the amp? directly to the speakers in the door going through the wire boot or do the normal hook up, but run the wire from the amp to the factory wiring harness? The HU is a deadhead with 3 pre-amp outs (2 fade and 1 nonfade)
Here's what I did when I installed my new speakers. Because my car had more of a wire plug in, I basically installed my amplifier in the trunk of the car... like many do, and then ran all new wiring 14 gauge to each of the speakers. But I was pushing 100 watts at 4 ohms. The thing about the factory wiring harness wires are that they're tied up next to power wiring(noise) and they're a pretty low quality gauge with a low strand count. Its a better quality if you were to go with some new speaker wire, wired directly to the amp. I'm not sure if it'll be more work for you, but it'll be worth it in the end.