Bobros

My trailer has a split tongue and this is what I found when opened up. The four pin is sealed and this is where I can view the wires. There are three whites, two browns, a green and a yellow.

These two at my thumb go to the side marker. The green and yellow are the taillight turn and brakes.

This one at my thumb turns off the tail lights when disconnected. Note the fuse on my truck that was blown was for the taillights. The brake lights and turn signals continue to work.

Will have to double check this one.

Steve83

Unplug them all, then start testing continuity (resistance, diode check, tone when the meter's probes touch each other or the same piece of metal) between the terminal for EACH white wire, and that white wire's other end. Since most of the lights are working, I expect you'll find continuity between all the whites going forward through the trailer neck; and going rearward through each light's white wire, except one of those with a brown wire - the one that blew the fuse when it got pinched or chafed.

Bobros

Quick report, I gave up trying to fix it and now replacing the lights and running new wire with the typical grounding scenarios. All is working. Tc for all the input. I did find a YouTube vid with a very similar condition and his issue was in the plug with two wires mixed.

Flamingtaco

Next time you have an issue, do as Steve83 suggested. With the trailer disconnected, put the meter on ohms and check each contact against all the others. Resistance should remain at millions of ohms if there are no shorts.

If you have a continuity setting, it will have a diode like seen in white below. On this setting it's looking for a drop of 0.6v or less and will beep if it does. Any short through wiring will drop less than that, so it's an easy way to test for one without having to look at the meter.