Connect it to the trailer chassis. Anywhere is ok, just make sure it is connected to solid unpainted metal.
Please get a multi-meter and check resistance at the point where you connect ground wire from the towing vehicle and the points where you would connect the grounds for the lights. If the resistance are too high better use direct wire. I bought a folding trailer from local Harbor Freights and I had to use a ground wire because there were no conductivities in the frame.
poor ground wire
Fuse is blown light connector is unplugged ground wire on trailer disconnected
Sounds like a weak ground connection somewhere on the trailer or tow vehicle.
#1 reason is always bad GROUND. Now you really weren't all that specific about which plug. Trailer plug on the vehicle or actually on the trailer? If it's the plug on the vehicle, I would strongly suspect bad ground or installation was backwards. If on the trailer, merely backwards installation. In any case, get better trailer light performance by running separate ground wires to each individual light socket on the trailer and connect them to the ground wire at the plug so they will be grounded to the vehicle at time of connection. I do this with all of my trailer lighting and never have issues.
Vehicle side, Brown, running lights Yellow, left signal/brake light Green, right signal/brake light Black, ground Trailer side, white is ground
check the ground wire on the trailer. if it is corroded at all they may not work. take a file to where the ground wire is hooked up to make sure it is on bare metal
is the tow haul light flash when trailer is plugged in/ if so this means you have a bad connection. usually this is fixed by pushing the plug in more securely . this can also be caused by a bad ground between vehicle and trailer at the towball. easiest way to fix this is to add a ground wire to trailer plug
grounding. Almost always an inadequate ground for one or other the light assembly or most commonly the ground between trailer and tow vehicle (tongue ground is not enough) - inadequate ground can work with low current loads and behave as if dead circuit with the higher load of additional lamps.
Double check that you have a good ground on the trailer and on the vehicle. White wire should be connected to bare metal. Also ensure your trailer tail light bulbs are not twisted in their sockets.
You can use "Scotch locks" on the tailights, and stop lights - trace them from the rear light fixture
Check for ground (most cases bad ground is the problem) Check for power getting to light sockets Check the bulbs
There is a short to ground in the circuit. You will need to look for a wire that has rubbed through to the frame or body. Be sure to inspect wiring to the trailer light plug if equipped.There is a short to ground in the circuit. You will need to look for a wire that has rubbed through to the frame or body. Be sure to inspect wiring to the trailer light plug if equipped.