You might find more help over on the Ford forum and give them the make and model.
Generally speaking many tractors that have ammeters generally feed loads like lights and ignition etc from the Load (NOT battery) side of the ammeter. A wire from there could typically feed the BAT Input terminal on a light switch while the light switches Lights output terminal (hot if switched ON) then wires to the light or lights. Some tractors fed loads instead from the LOAD terminal on a Voltage Regulator that had that extra LOAD terminal. Often a fuze would be in the line located near the source voltage. Often the lights use the tractors conductive metallic frame (ground) as the return current path that way only one hot wire is required from the switch to the light and the frame serves as the other (non wire) current conductor back to the battery which also has one of its terminals tied to the tractors metallic conductive frame. John T