Sounds like you have a cut wire or missing ground somewhere. Typically those lights are wired using a single wire down each side. In most cases the wire from the umbilical has a splice right inside the trailer, one line will lead to the rear tail lights, the other feeds the running lights, I have also seen the splice at the back near the tail lights too.
Also just of giggles and grins... make sure they have bulbs in them. I had a trailer one time with no interior lights, checked fuses, started on the wiring, pulled a cover to get a continuity check and no bulbs, pulled another cover and another...