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The wiring harness on the front of these vehicles commonly develops corrosion in the area behind the left headlamp, behind the bumper. This can cause all sorts of lighting related issues, depending on the wires involved. If you find any corroded wires, or green spots, cut out that section, and solder in a new section, being sure to seal the connections with heat shrink tubing.
Another distinct possibility is a faulty headlamp switch or dimmer switch.