No Trailer Lights When Using Tow Vehicle 2014 Volkswagen Tiguan  

I have a 2010 VW Tiguan. I had the dealership install the wiring 3 years ago on the vehicle side so I can tow my boat. They installed a module that runs down to the bumper with a mounted cylinder adapter receiver. I then plug the 4 pin from the boat trailer to the cylinder adapter that plugs into the bumper. My trailer lights no longer work. I took it to the dealership and they plugged a cylinder adapter with lightsa tester into the bumper. All of the lights lit up when going through each test. I thought as well as the dealership that perhaps my 7 pole to 4 pole adapter was faulty. I bought a new adapter from Pep Boys and went home to test. Trailer still didnt light up. I also purchased a tester that plugs into the adapter which showed the lights blinking on the adapter. The adapter lit up for signal, brake lights, running lights. A few days before discovering the issue with my boat, I went to Uhaul to rent a trailer. They wouldnt let me drive off the lot because the trailer lights were not working. They assured me they test them regularly. So my boat trailer wouldnt light up nor would the uhaul trailer. I had my dad come over with his truck and it did power up my boat trailer! but not all lights were working. I dont understand why the adapterwith the tester plugged in gives flashing lights which tells me power is going through the adapter and working properly, but when I attach to 2 different trailers, nothing works, and my dad hooks up his truck and they work. Super stumped, hoping you guys can help!!

asked by: Andy