Most newer trucks have seperate fuses for the trailer circuits, and sometimes they are in their own fuse holder, it might help for you to let us know what truck you are working on. Also does the truck have seperate bulbs for the turn signals vs stop lamps. If so , it might not have been wired up right or the bulb integrator has failed.
If the actual ground wire (white) has not been hooked up on trailer and truck, then the trailer would have to rely on grounding through the trailer ball, I see this a lot and it usually causes intermittant lamp operation and flickering lights.
If the actual ground wire (white) has not been hooked up on trailer and truck, then the trailer would have to rely on grounding through the trailer ball, I see this a lot and it usually causes intermittant lamp operation and flickering lights.