Look at the diagram above. The red wire on your trailer is not passing electricity from your trailer battery. Or the negative wire is not letting the electricity back into the battery.
- If you have a disconnect switch make sure it is allowing power through the switch. You can check by turning on an interior light
- The trailer battery is dead. Verify by turning on an interior light
- The trailer battery negative wire is not hooked to a good clean bare spot on the trailer frame
- Less likely, your truck and trailer wiring are not standard and you are just using the wrong terminals
Do you have a multimeter or a 12 volt test light?