I posted this in the trailer section then seen it may better fit here,

Here goes, I just purchased a used enclosed snowmobile trailer the lights are not working correctly. The trailer and top both have lights so there are a total of three bulbs for the left and right brake and turn signals, two on the top and one on the trailer.

I have a tester I plugged into the tow vehicle '07 explorer with factory tow package, the running lights, left and right turn signals and brakes lights all tested good plus it has always been fine with other trailers.

When I plug in to the trailer I have running lights but no brake or turn signal lights, I disconnected the lights on the trailer then the topper lights worked fine, I then disconnected the topper lights and the trailer lights work fine but with all them connected they won't work. I did notice after turning on the turn signals and trying the brakes with all the lights connected I then plugged in my tester and it was not working for the turn signals or brakes, I turned the key and lights off then back on and the tester was working fine again.

Sorry for the long post do you think a bad ground on the trailer and during higher current the tow vehicle is detecting a fault turning off a relay or something until a key cycle?

Thanks

I did one more set of tests...

1) Turned on the running lights to the trailer, let it sit for a few minutes then disconnected the trailer and hooked up the tester and everything was fine.
2) Connected back to the trailer turned on the right blinker nothing on the trailer, tow vehicle is fine, unplugged the trailer and connected the tester and the right turn and right brake didn't light. Cycled power and tested out find, did the same test for the left turn same results. Then cycled power and applied the brake, hooked up tester and both left and right were out until the next power cycle.

 

I would suggest taking the trailer plug apart, and use a small battery charger, put power to the trailer lighting wires and see what you get. Look up what color wire should be what function(you can find that on the internet if your wiring is standard) and put one clamp of the battery charger to the trailer frame, and then touch the other to the proper wire, and see if the proper bulbs for that wire light. Then keep using the battery charger for testing out the wires on the trailer till you figure it out with a meter or a testlight for troublshooting on the trailer while the battery charger is still powering the trailer.