mitchgates

I have a '93 Maxima (non-bose) in which I have installed a new Alpine headunit and 4 new pioneer speakers. I bypassed the rear factory clarion amp with simple splicing of the exising wire in the trunk. However, the factory wires that go from the dash to the rear appear to be very small gague and NOT copper...so I assume I am loosing signal quality. I want to run my own wires from the H.U. to the back speakers, but have no idea how and were to feed them. What is the least complicated way to do this that will not result in exposed wiring?

Thanks,
Mitch