Those Harbor Freight trailer uses a grounding screw near the tip of the trailer for the ground from the car and all the grounds for the lights will tap into the ground via the trailer frame. The ground became too weak I guess by the time it gets to the tail light.
The solution was I connect one ground wire each from that screw to the taillights and it worked. The tricky part was when my day time running lights are all off during the day, all the trailer lights actually works even without the added ground wire, but when the car’s running lights are all on (during night time), the tail light will simply not work without the add ground wire…
They should have told you to run the freaking ground wire to the tail lights to begin with…:retardclap:retardclap