7-Way Trailer Wiring Harness

Used this to wire a 26' car hauler with tandem brake axles. Everything went well but trailer brakes stopped working during second pull, NOT GOOD. After testing from the plug I found power at first junction box that was supplied buy MICTUNING Heavy Duty 7 Way Plug Inline Trailer Cord with 7 Gang Junction Box - 8 Feet, Weatherproof. Then I checked the second junction box at the axles that was supplied by this wire and could not get power. Double checked the connection in the first box and wire had power at the connection terminal. Somewhere between the two boxes wire has an issue. I confirmed that by running external jumper wire from box to box and brakes work. I spent extra time and money buying the junction boxes and this wire and running everything nice and neat with wire loops and then the internal wire fails. This wire seems good for the lights, no problems yet, but for the brakes I would not recommend it.