Tracing out light problems can be a bit$h. I have spent several hours tring to get the lights on my M151 to work again. Turns out I probably had a short that fried my three lever switch. My replacement switch did not work also. I took a switch out of my duece today after I saw it work the lights and installed it in the Jeep. Still no lights. Turns out either the high beans or low were not working so all my testing still did not work. When I switched beams it worked. I now have all lights working. I have learned quit a bit so maybe I can help you. First you have to be sure you have a hot wire coming in. When you unscrew the big coupler from behind the three lever switch you will see a locater pin at the bottom. Counting from there clockwise the hot lead is the third pin. Check to see if it is hot. It should be hot all the time. If it is then the pin inboard to the right is the headlight pin. Run a jumper to it and see if you have lights, they shoud work in low and high. If that lead is hot and the lights don't work then you have a wiring problem after the switch. If they do work then the switch could be bad. Check continuity between those two pins. My two switches that were bad had no continuity. You can take the switch apart and clean the contacts and maybe make it work. But first smell it and see if it smells burnt, mine did so it was fried in a place that is not accesible and not repairable.
