simply cut the wires going to the rear speakers, twist the ends and clamp them in a butt splice and then clamp that corresponding wire for that reQ into the other side of the splice?
i'm doing my best to spell everything out before i start cutting wires
Not commenting on the electrical side...
This is a nice option for tapping into existing wiring. The continuity of the wire remains unbroken and the voltage / current is branched to a new location.
Ensure the splice is sealed from moisture ingress. This is important! Right Splice Leaves Factory Wiring Unbroken.
A IDC crimp splice is another option. It just leaves two small slits in the insulation.