I finally have got enough time to look at my trailer and tried to find out why electricals were [mostly] not working. I did some standard troubleshooting tricks and found out that I have a bad ground connection. When I cleaned up an area of the trailer with an angle grinder and hooked up a 12v negative to that, instead of white on the plug, the lights lit up.
This trailer uses the body of the trailer as a negative ground. I have a feeling that it does not work well after rust sets in.
I would like to ask if it makes sence to just redo it all, rip out and throw everything away, and make new wiring.
Since I have forklifts, I can easily turn the trailer to the side or upside down.
i