If you don't use the rear wipers and have the button taking up space, then this is for you. Be sure to unplug the rear wiper motor before doing this mod. Also do not hook up high amperage loads to the switch. Run it through a relay.
To have the switch output a positive voltage splice into the blue/red wire.
To have it output a negative voltage you need to cut the green/red wire and connect the switch side of it to the black wire. In the end it should look like this