Since you already have to run power wire, RCA cables, and a remote turn on lead, just run 4 pair of speaker wires as well and disconnect the stock speaker wires from the aftermarket harness and splice them there. Use butt crimp connectors or solder & shrink tubing. You can just rewire the rear speakers straight to the amp as well, leaving them connected to the harness and simply unplugged at the speakers with new spade terminals on the new wire.