The fact your tail lights are working sure makes me think the headlamps have simply burned out. The head and tail lights are the last in the wiring chain. Look under the fenders behind the headlights. Remove the connector that goes to the headlight. See if you can measure 12 volts from the tractor cable end. Doing this first will confirm the headlight condition before purchasing new ones. If you do not measure 12 volts then I can look further into wire locations for places to check. If you do measure 12 volts, then it is time for new bulbs.