it sounds fine if you don't run new wire to them. but for best quality, i recommend running new wire to them.. as for the wire connected from the head unit to the factory speaker wiring, that shouldn't be needed if you are connected to amps through preouts. it doesn't do anything at that point.

its up to you if you want to run the wires to the speakers, it's probably recommended to for best quality, but not necessary..

where are the amps mounted? if in the trunk, it should be fairly easy to run wires to the rear deck..