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The headlight bulbs are ground controlled. It sounds like you have a bad ground circuit. The bulbs get power from two separate fuses and the headlight switch controls the ground of the high and low beam circuits. Both headlights share common circuits for high and low beam, Check at the right head light and you should notice two of the wires share the same color with the left side, If the right side works and the left doesn't you could run wires from the right light and over to the left joining the same color wires at both sides and it should work .