The switch isn't the problem. It's turning on when you shift to reverse and putting the short in the circuit. A simple trick is to use a pair of spade terminals and jumper wires to plug in a 12 volt light bulb in place of the fuse. The bulb will be full brightness when you shift into reverse with the ignition switch turned on. That shows the short is there. The bulb will limit current to a save level.
Now you can move the wiring harness around in the trunk and unplug various things. When you isolate the short, the bulb will go out or get dim.