- The wiring diagram for a 1951 Chevy pickup is located at: http://chevy.oldcarmanualproject.com/electrical/wiring/pdf/51truck.pdf
- Other wiring diagrams are at: http://chevy.oldcarmanualproject.com/electrical/wiring/index.htm

Based on a quick view of the schematic, the dimmer switch could be the prime suspect (assuming the the wiring is still stock) since it is common to both functions. Try disconnecting the beam feed wires from the dimmer switch and manipulating them manually to the battery feed alternately.
- battery= black and red wire (you can leave connected)
- low beam feeder wire= Green wire (short to black and red wire for test, then disconnect)
- high beam feeder wire= Black wire (short to black and red wire for test, then disconnect))
If it is working manually, it is most likely the dimmer switch
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