Is it possible to unplug the amp and connect the wires that go in to the ones that come out, instead of rewiring the whole car? I would think that would be what the "bypass" harness does.
My new radio sounds alot better than the stock one, btw.
I don't think you would have much sound coming out of it the speakers if you did this alone with the factory stereo. With an aftermarket one, not sure. I actually think its easier to rewire the whole system, the amount of wires coming out of the factory amp is enough to make an electrician think twice.