I have a harbor freight tag-a-long trailer and having difficulties with getting the lights to work. I have an Addon trailer wiring harness w/isolator relay. I can verify that this works by using an ohm meter. THe issue seems to be with the 5 to 4 wiring harness convertor. The running lights work, but no brake lights or turn signals. I have tried 2 different types of convertors, a Hopkins and a U Haul brand, both with the same results. Any ideas or suggestion on where to look?