I drive a 2013 Outback, and have a 1996 Jayco Eagle 8. My car only has a 4 way trailer harness, and the trailer has a 7 way plug. The owner before me only had a 4 way vehicle, so he added a 4 way plug to the trailer, and literately zip tied brake lights to the rear of the trailer. I want to use the trailers lights, so I bought an adapter, but I am having issues. The front lights on the trailer work correctly, but all the lights on the rear of the trailer do not. I took a bulb out and swapped it with the front, and the rear still did not work, and the front did, so it's not the bulbs. I am getting about 5 volts to the brake lights when the vehicle brakes are hit. I'm not sure if that is enough or not. How do I go about trouble shooting from here?
Also, the trailer does not have brakes, and I am not concerned with a 12v hot wire, unless that is the issue, so the three wires coming off the adapter are just there. I did ground the adapter and it has continuity to the negative terminal car battery. Also, the trailer has a battery and it is hooked up.