Aftermarket switches are grounded when switched on, basically creating a short, and should not be used without re-wiring and using a relay. The OEM switch passes through the 12vdc power.
To make the relay work you will need to cut the yellow and red wires before the reverse gear plug on the harness side.
Connect the Yellow wire on the harness to both pin 30 (power) & pin 86 (trigger +). These will always be hot when the key is in the run position.
Connect the Yellow wire on the harness side of the plug to pin 85 (trigger -). This wire goes to the reverse switch and when the switch is engaged, the coil in the relay is triggered because it is now grounded by the aftermarket switch.
Connect the Red wire on the harness to pin 87 (power out). When the relay is triggered, the 12vdc from pin 30 is the switched on and passes out pin 87, and your reverse lights should come on. The red wire on the connecter side is not used and should be taped off.
If using the oem switch and you want to put your lights on a relay or have the ability to turn them on on demand, take the red wire on the connecter side and just ground it to the body. This will do the same thing as the aftermarket switch that seems to be available at the parts stores.
To test to see what kind of switch you have, take a volt/ohm meter and switch it to the 200 or so setting to test continuity. Place the + probe on the + wire of the switch, and the neg probe to the body of the switch that would be grounded to the transmission. Now press the button on the switch. If you have continuity, then you must rewire with the relay. If you just have continuity on the - wire of the switch, then it would work like the oem switch. If the aftermarket switch button is metal, then there is a high probability that the switch is a grounding type of switch and will short out on a Samurai.