trailer tow lights not working how to troubleshoot

Welcome. We are going to need a picture of the thing behind the cupboard next to the glass fuse... LOL I don't think it has anything to do with the tow wiring.

Tow wiring is easy. We have several tutorials here on how to test. Check the stickies at the top of the board in this section. Do not use your tow vehicle to test the trailer wiring. Eliminating the tow vehicle makes it much easier to troubleshoot the trailer.

vintagetrailertalk.freeforums.net/thread/11040/testing-trailer-wiring?page=1&scrollTo=20553

First, check that there is a GOOD GROUND on the trailer frame. There should be a wire connected to the frame. You may have to clean the connection and/or add a new bolt. This is the most important wire of the tow wiring.

Second, remove each bulb and clean the light socket and the light grounds. The lights are usually grounded to the skin. If the screws attaching the light plate are corroded, the light is not grounded and won't work. You can use a jumper wire from the plate to the trailer frame if you need too. Remember, the GROUNDS are the most important wires.

There are only 3 wires that operate the tow lighting:

TM: this is your tails and marker lights (clearance lights) Some vintage trailers use a Green wire.
RT: Right Turn some vintage trailers use a Brown wire
LT: Left turn some vintage trailers use a Red wire

Many vehicles and most vintage travel trailers do not use common colors so you will need to use the testing process described in the Testing Trailer Wiring thread. The battery will be grounded to the frame. Use a fused wire from the battery hot to one of the trailer wires. One or many of the lights should glow. Label each wire according to what light glows.

I hope this helps.