Did just that, and its reading low voltage on the wire. Going to have to replace the wiring harness.
Thanks all for the advice.
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Remember back in the day all you had to do was find the wiring harness in the tail light assy, strip back the insulation and tap in that wiring harness from Canadain Tire.... took maybe 30 minutes and cost about $5. Fuses ??? HaHa..
Best tip I ever got for trailer wiring was ground it to the truck. Avoid all this frame grounding nonsense. Run ground straight back to your wiring harness and thus back to the truck itself.
I had the exact same issue in my old boat trailer. My left side running lights were not working - but I had signal and brake lights!!??
Being an electrician this drove me nuts so one afternoon I ripped it apart to find the culprit. Here was my order of operations
(1) check your truck to make sure your plug is working
(2) check the trailer connector
(3) check the condition of the cable from the boat plug to the light termination at the back of the trailer (make sure the wire is not grounded or broken) - check the terminations at the lights as well.
(4) check the ground connections at the lights - if you have a folding tongue on the trailer check the jumper cable at the hinge
(5) check your lights
The issue in my trailer was a bad LED driver in the left light for the running light circuit. LED lights are very robust but not bullet proof - yes they do fail. When wiring your trailer please do NOT use butt crimp connectors - they will last for 1-2 years but will ultimately fail on you - even if you heat shrink them. I prefer to strip the wires long enough to twist together then twist back on one of the leads then solder them and then heat shrink them - just remember to put the heat shrink tube on BEFORE you solder the wires.
Also some abrasion resistant tubing or material when the wires enter or exit the trailer frame goes a long way.