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On a military 416 trailer. Only thing is, the guy converted it to 12 volt. How difficult will it be to convert it back to 24 volt?

I wouldn't let a good lead by just on that issue. But as luck goes, who knows what can happen.
Could just be the bulbs. Is the stock wiring still in place?

If the wiring and light housings are original and in good condition you can use them with either 12 or 24 volt. The bulbs can be switched out in the original housings to go from 24 to 12 volt. If the wiring was switched make sure it is as least as heavy as the original if you are going back to 24 volt and you won't have any problems. I made a pigtail with a spare jeep trailer plug on one end and a adapter to fit my F150 on the other. Turn signals and stop lights work fine. But when I'm pulling the trailer with my truck I don't have convoy lights, of course.

skyjeep50 wrote:If the wiring and light housings are original and in good condition you can use them with either 12 or 24 volt. The bulbs can be switched out in the original housings to go from 24 to 12 volt. If the wiring was switched make sure it is as least as heavy as the original if you are going back to 24 volt and you won't have any problems. I made a pigtail with a spare jeep trailer plug on one end and a adapter to fit my F150 on the other. Turn signals and stop lights work fine. But when I'm pulling the trailer with my truck I don't have convoy lights, of course.

I've been thinking about doing this. Are you saying you switch bulbs depending on which vehicle you're using to tow?

The 14 gauge wiring used by the Army is adequate for trailer lighting at 24 volts, 12 volts and 6 volts. It was a case of overbuilding by the Army to use such a heavy gauge wire. Keep in mind that for the same amperage load the higher the voltage the lighter the wire gauge.

The lamps were the only component of the jeep type M series trailers that were voltage sensitive.

1683 - 24V, 1156 - 12V, 1129 - 6V
1251 - 24V, 67 - 12V, 63 - 6V

Thanks for the part numbers Wes - I couldn't find my 12 v bulbs - lost somewhere when I moved my shop recently. Actually, I have used the trailer with the 24 v bulbs and a 12 v truck. Yes, they are dim and it isn't a good idea but short hauls on country roads, not a big problem.
Here's the adapter - spare trailer socket and store bought standard 4 prong trailer plugs.

Nifty.