They are hooked up to the van reverse lights through a switch which also overrides the lights to on.
A SPDT ON*ON switch with the center wire going to the bumper lights and one outside wire connected to the van reverse lights-on 12v, the other to 12v.
It's why there is no light on the SMB reverse switch, because one way connects the lights to the van reverse lights and the other way turns them on manually, while simultaneously disconnecting them from the van reverse lights.
Somehow I managed to put a lighted toggle in place of the SMB switch (after driving for Â½ hour with them on) and have it all still work. I can't remember what I did, but it's probably in my switch install thread.