BenK wrote:
Body grounding methods abound with El Cheapo...
The mounting nut might not be tight...or touching with a high resistance
The tab that is supposed to touch body ground may no longer have proper spring tension...therefore poor connection with high resistance
Dirt/water/etc gets into the contact area to corrode it and make a high resistance connection
That list goes on and on
Proper grounding is a dedicated 'wire' with proper UL/CSA/etc approve wire and connectors...along with approved methods on installation. That all costs more than an El Cheapo pushed on light with a spring clip to hold it in the hole...that also serves as the ground point...some times that spring clip is supposed to knife through the El Cheapo paint on that mounting hole edge...some times it does, some times is doesn't...some times it only kinda sorta does...
Most intermittent connections comes from body grouding schemes...
Again, agree 100%. In this particular case, the OP has a working brake light within the same fixture as a non-working tail light. They usually not only share the same ground, they are usually the exact same bulb with 2 filaments. That means the ground (in this case) is not suspect.
Including my response to the OP's duplicate post in the TT subforum since it seems to be fading away.
wnjj wrote:
The tail lights and all marker lights are from one pin on the trailer plug. Since you have at least one working, the truck, plug and trailer cord are fine.
Start by following the trailer wire cord from the plug to where it breaks out into the various circuits. There may be a splice block where wires from the front and rear marker lights join together with the trailer cord. If not, then there is something wrong inside the trailer wiring. You could pull each marker light, looking for a failed splice. I'd start with the left tail light since you're seeing "voltage" there (how much?). Worst case you could run a new wire from the trailer cord to one of the non-working lights and they should all start working.
Also, if you see 12V at the socket with no bulb installed but it doesn't light when you replace the bulb, that likely means a really poor connection to that socket, either right there or where the wire gets its power.