Unplug both low beam headlight bulbs.

Take a test light and see if you have any power to the bulb socket when it's on "low beam".

If you have power, then you have a poor ground, (maybe even a burnt out bulb(s).

If you don't have power at the socket plug, start tracing it back to the headlight and dimmer switch until you find the open.

Headlight switches have circuit breakers inside that can fail.

Don't overlook the plug at the firewall were both main harnesses plug together,