I just used the original wiring from the factory. Got one of those Scoche wiring harness adapters and crimped it onto the aftermarket head unit. Left everything else the way it was.
I did have to replace my speakers as the passenger's front one and the driver's rear were both blown out bad. But again, used the factory wiring. Just crimped new connectors on the speaker ends and called it good.
No offense, but unless you're going in to audio competitions where you're running several hundred watts into your door speakers, the factory wiring is going to be more than sufficient. 18/20 gauge wire will handle more than anything a standard head unit can put out. Really, why re-engineer something the factory already did for you?
And since the factory amp is already there, why not make use of it?
I've got 50W Blaupunkt speakers in the front and 55W Pioneer speakers in the rear. I can take my head unit up to just before the distortion point and all the speakers handle it just fine. To the point that even screaming at each other you're not going to have a conversation. Just ain't happening. The stereo is too loud.
Add in the 160W home stereo sub in the back and there's no way in hell you're seeing anything out of any of the three mirrors. They're just a blur.
Oh, and none of my levels are cranked. Bass is 50%, treble is about 80% and the sub is at 50% (with the gain on the sub at 50% and crossover at 100Hz). Bass doubler setting is off as is Loudness.