I was checking my dimmer switch just the other day and determined that as I switched it back and forth I would get continuity from the power in terminal to only one power out terminal at a time. This leads me to believe that on low beam one wire is hot and on high beam the other wire is hot. Not both at the same tine.

See Oehm Jr's 46 answer to "More on Headlight Wires" Topic. He has a nice diagram of the headlight plug


1941 Chevy Master 3/4-Ton
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Born to fish, the truck is only a hobby.