I have a 2002 Toyota Tacoma Pre-Runner where my low beams don't work but my high beams do work. I've replaced the headlight bulbs and relay switch, and still the low beams don't work.
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According to the wiring diagram the power is supplied to the bulbs through the relay and then one fuse for each bulb. If the high beams work then the relay and fuses are working. The lights are controlled by the light switch by switching the ground side of the circuit. The high and low beam filaments are selected by this switch. If one set of the filaments is inoperative look at the ground circuit and the switch in particular.
I have had similar issues on an older 4runner. After trying a lot of fixes, eventually found that the fault was in the wiring harness that runs under the steering column. The hot 12v that eventually feeds both headlights (or relays) had degraded in the connector until there was no longer continuity. I had to pull the wires from the harness and splice them manually.