There are multiple fuses that operate the 7 pin connector. Running lights fuse for trailer, brake FUSES depending if you have a brake controller and turn signals all on different fuses. You need to see where the two leads tie in and see where the connection is lost if the 4 pin is working correctly and you are sure of it! If the two options are made together you may have a faulty combo plug. You will need a test light or multi meter to help.
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