Have you validated that there truly is no 12 volts to the switch with a meter. A common problem with lights SMB adds is they attach the ground to the body. That screw comes loose or the connection gets corroded and you no longer complete the circuit, but the break is on the return path back to the battery.
If not that then I would suspect a bad wire spice to the (I assume) passenger side 12 volt line. SMB was fond of the quick splice connectors
on my rig.