I have a 1966 model 4020 diesel with the 24 volt charging system.The lights worked fine a few weeks ago.Now the inside fender lights and the rear light don't work.The outside lights work fine.I pulled the yellow wire off the back of the light switch that controls the inside lights,jumped voltage from the negative hot post of the battery and the lights work.The spade on the back of the light switch is hot,but when I plug the yellow wire back up the voltage goes away.I touched a test light to the bottom of the spade,the light comes on.When I plug the yellow wire in,the light goes out.I replaced the switch,but it didn't help.There are two relays below the switch.If I put voltage to the lower one the lights will work also.I've checked the battery ground ,positive to frame.It looks like a grounding issue to me,but I don't know where else to look.