Originally Posted by ts8501
All helpful, but let me add that I am currently indoors on a concrete floor. I am thinking I should attach the trailer to the TV, and drop one of the chains from the trailer to the floor. I must have enough 12v
ground by then I would hope.
The kind of ground you need has nothing to do with the floor or dirt.
Look at it this way. Every electrical circuit needs a way to get power TO the load (light) and power BACK to the power source (battery).
This means every circuit needs 2 wires, one to the load, one going back.
Somebody cleverly figured out, that all you really need is a piece of metal in place of the wire going back to the battery. And guess what, your car is made of metal! Also your trailer is made of metal!
By using the car or trailer in place of a piece of wire, they make your wiring much simpler and save a fortune in wire.
When they do this it is called "grounding" the wire, even though no actual ground or dirt is involved.
I hope this is clear.
In your case, if the trailer lights are grounded to the trailer, and the power comes from the car, you have to have the trailer and car connected together to complete the circuit.
When you plug in your trailer lights this should happen automatically because one of the wires in the harness is a "ground" wire that connects your trailer to your car.
If this is not happening you need to check the ground circuit in the plug. For a temporary test you can connect a jumper cable from your car frame to the trailer frame. Be sure the connection is on bare metal. Any rust, dirt or paint can act as an insulation.
Just hitching up will usually create a ground. But this is not always certain especially if you grease the ball. So they add a ground wire to be sure.