Here's an intersting one. I was looking at the wiring diagram for the 1945 GMC 1/2 ton with 6 volt positive ground system. Within the diagram, (from the original owners manual) they show a "12 volt system". It shows the ig switch with a lead to coil, a lead to ammeter and another to an additional section. (wish I could post this) there are 2 resistors and then the guages. How's that do that? Where's the 12 volts coming from? must be similar to AD's.