Do you have separate fuses for high and low beam? If so then check them. If you have a separate low beam fuse then swap it out no matter how it looks.
If you have the passing trigger - does the low beam come on with that? If not - then check your wiring connectors to make sure you don't have a loose connection. Trace from the headlights to the battery and to the handlebar switch. If it does -then skip the wiring because it's the contacts in the switch.
If the wiring was fine then the next thing to check is the switch. You can take it apart and look at it, then clean it with electrical cleaner. If it looks like contacts are discolored then they aren't making good contact and cleaning them and lubricating with electrical grease may cure it - but you may need to replace the switch.
If this doesn't find it - then you are going to need a multimeter or at least a continuity tester.
When checking the connectors and wires - look for any hot spots (discolored connectors or wires) because they indicate a problem.
Good luck.
