Originally Posted by 55 Nomad
Yes it is that light. I measured voltage at the input side of the switch. Also at the splice about 4" before switch. That is a good point about the switch as I didn't think about that.
Good practice when troubleshooting 12v systems. Remember that every fixture has two current wires, the 12v “line” and the “ground”. Most people assume that a voltage problem is the line voltage and simply use the ground point on the fixture as the other connection. But in an RV it is at least as likely for the ground to become disconnected through corrosion or some other detachment from the path of conductivity.
In your case, I suggest using a known ground point for your meter, something metal that for certain has a conductive path to the vehicle chassis. If you then get a 12v reading at from your supply line at the fixture, then you know the ground is the problem, not the supply. A low voltage reading at your light fixture would definitely point at a corroded ground contact. And much easier to repair, if you can’t find where the discontinuity is, you can put another screw through to metal if grounded or run a new wire to the nearest/easiest ground point.