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My driver's side headlight blew out. So I bought a new one and installed it. Packard connectors, not Douglas shell, but so was the one I removed. I made sure I lined up the three connectors with the old lines, and plugged it all in. It worked great, for a day. Then it suddenly stopped working. It was freezing out, and I figured I'd do a circuit test in the morning. Then, as I drove home, I noticed the headlight was on again. This morning, I turned the lights on, and it was off. I pulled the headlight, and tested the three wires with the light switch on. One had current, two did not. I think one goes to ground, are the other two supposed to be live?

Are you talking about the wires coming from the headlight? Circuit 91 is the ground, 17 and 18 are high beam and low beam respectively. I had a similar problem with blowing of headlights in my jeep. Check the voltage in the wiring harness at the headlight. Mine was registering high so I adjusted the voltage at the regulator.
Kendall

Alright. Circuit tested everything. All the correct wires get power, and the highbeams work. Only the low beams don't. I tested it on a voltmeter, and it doesn't get too much voltage. Low beams must just have blown.

As already mentioned excessive high voltage will burn out you lamps. Since you have the voltmeter in hand why not start the engine, accelerate it to about the RPM equivalent of 40 MPH and turn on the headlights. If your reading is above 28 .5 volts then I suggest you adjust the regulator for 28.5.

If the reading is 25 to 28.5 volts then she is fine and the issue is in bulb quality. Have you taken ohm readings of the bulb's filaments to see if they are actually blown?

How do I take the ohm readings Wes?

That's a lot of typing to explain and depends on the meter you are using and it's ranges. It's time to get a helper who knows electricity and meters.

This may be a stupid question but where did you check voltage
on the wiring? If at the headlights I respectfully walk away.

The reason I ask is if the voltage was not checked at the headlights
there could be a little confusion and the dimmer switch may be bad,
providing voltage to the high beams only. That also may explain
the intermittent operation.

Those old switches are mechanical, and after having one apart
can easily see the thing failing partially or completely.

I checked voltage right where the packard connectors mate next to the back of the headlight bucket. I also checked them inside the headlight bucket where they mate to the cables extruding out of the bulb itself. I revved the engine and it didn't go above 26.5 volts.

What was the bottom end voltage at idle? too little voltage can cause some issues as well.

25.5

So, Wes turned out to be the closest. A metal terminal inside a packard connector had backed up in the sleeve and wasn't fully connecting with the male end most of the time. Everything turned out to be in working order, and now I have an extra headlight just in case.