If no lights
are working, it is almost certainly a ground problem. There is usually a white wire that screws onto the trailer frame. Check the connection -- it is easily broken. If you are not familiar with vehicle electrics, the chassis (frame) acts as your connection to the negative side of the battery
. All of the other wires are positive going to the lights.
Typical "flat four" wiring is as follows:
White = ground
Yellow = left turn/brake
Green = right turn/brake
Brown = tail lights
(all marker lights)
The way to use your tester would be to work backwards from the tow vehicle and see where you aren't getting juice any more. Start at the flat four in the TV and see if the male connector (white wire) has continuity to ground. The next connector should be the brown wire, and should be hot whenever the TV lights are on. The other two will flash with turn signals and brakes
If you aren't getting ANY signal from the flat four of the TV, then a fuse is burned out or your lighting
converter is fried.
If you have a good ground and good signals on the other three when appropriate, then move on to the trailer itself. Again, if you have no lights, your ground is the most likely culprit.