Hi Ki,

Check the ground wires in the headlight harness for continuity and that the
grounding point under the main fuse panel is clean, shiny and secure.
As the other post suggests, could also be the ignition switch.
 Easy to check if you have a new or known good switch. Uncover the ignition
switch, un-plug the ignition harness from the existing switch and plug the
harness into the 'good' switch and use a screw driver to turn the switch to
the run position and see if the light work!
Good luck  Phil Z.