Customer replaced bulb to low beam on driver side, but it still would not work when lights switch turned on. - Solution - Replace the LT LOW BEAM 15 amp fuse with a new one and the light works again.
I would start with the low beam relay. If thats good, check bulbs for continuity and both fuses 10amp and 15amp . If you get no success with those next thing to check is wiring harness.