I finally decided to see if I could get my 16 foot trailer hooked up to my van, in regards to the wiring. I found a nice adapter that has both a 6 pole round and a 4 pole flat connector on it and screwed this to the trailer hitch. I plugged it in to the 4 pole flat that comes from the van and tucked the excess wire back inside the van. Still need to tie that up nicely with plastic tie-wraps.
Next, I had to struggle to remove a 6-7 pole adapter off the trailer end. Once off I cleaned up the 6 pole round trailer connector and plugged it into the van. With testing, the RIGHT functions work, but the left did not. Checked the bulb on the left side. Works fine. Looked underneath and saw a wire that was not connected. It was a little short so I soldered a little splice into that wire and put heat shrink tubing in place. With testing it still did not work.
So I set the tail light fix idea aside for a little bit and focused on the "marker lights". There are 8 little bulbs under red or amber plastic caps. All but 1 of these had broken filaments. I found replacements easily at the auto parts store. When plugged in ALL of them work. So, back to the tail light.
There is a junction box that the trailer manufacturer installed on the tongue. I opened that up and found a little mess. Wires that did not have wire nuts on them and stuff just shoved in. So I tried to sort it all out. When done for the day I had the "running light" working on the left side, but not the brake light, nor the turn signal. I was frustrated, so I gave up for the day.
Using a multitester I was able to see a strong signal for the functions on the RIGHT tail light and a strong "running light" signal on the LEFT. When the probes are touched with the Left turn signal activated, I saw a strobing of the needle, but VERY weak. So I may have a bad wire or ground. If it was a ground problem, though, wouldn't it be present for the right side as well.
It rained most of the weekend, so I didn't get to complete this project. But I ALMOST have that trailer working. Will be nice to have it done. The final phase will be to run a brake wire and install a brake assist in the van. The trailer already has brakes, which are never used due to no brake wire.