Sounds like you might have a ground problem with the right light, or dependant on what kind of truck you are pulling with some of the toyato and nissans have turn signal independant of the brake lights and the wiring from the truck could be the cause of your problem. Quick check to see if its a ground is to turn the tail lights on and the right turn signal on at the same time if both work then its not a ground.If wired the usual way the right turn signal wire should be green. Hope this helps