The EASIEST WAY TO DO IT IS THIS WAY.
From be hind the head unit take the rear right speaker line and connect it to the front right speaker line.
Then do rear left and tie together to front left.You dont have to cut any insulation, plus the stock wiring is more than enough to insulate the wiring.
Then the only thing you will need to do in the rear is run speaker wire from the speaker wire connecting the rear speakers to your front outputs, then run another pair of speaker wires from the rear speakers to the rear outputs.