How did you determine that the bulb is good??? Sometimes a bulb will look good to the eye; you may even be able to see that the filament is unbroken, but this does not always mean that there is no defect in the bulb. The only sure way to determine whether a bulb is good is to test it for continuity with an ohmmeter. On bulbs which have two filaments; each filament must be tested separately. The other thing you could do is to plug the bulb into the right side headlight wiring, and see if it works there.
If you prove that the bulb still works, then there is probably a fault in the ground wire for the headlight on the left side.
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