I would partially disconnect the connector going to the lamp bulb so you expose the terminals but so they are still connected to the lamp.
There should be three conductors- Low filament, High filament and common.
Put your meter black lead onto a chassis ground location.
Put the meter red wire on the positive battery terminal. Verify you read 12 volts or thereabouts. This proves you have a valid ground point.
Now turn on low beams. Poke meter red lead into each of the bulb terminals one at a time. One should read 12 volts, the other two zero. If none read 12 volts, you have a supply issue. If more than one reads 12 volts you have a ground issue.