I've searched this forum and asked Dr. Google but haven't found someone else with exactly the same issue I have with our recently-bought 2018 Airstream Interstate EXT GT. I need to tow a trailer that uses a flat-4 wiring harness (just tail/running lights, blinkers/flashers, and brake lights). The flashers, both blinkers, and the running lights work... but the brake lights don't. (With this trailer's setup, a single bulb/filament is used for blinkers/flashers and brakes.) I tried this trailer with our Honda Pilot (which has a wired-in flat-4 harness/plug), and it works fine. I tried a different similarly-wired trailer, and it worked fine with the Pilot but not with the Interstate. I've tried two different 7-to-4 adapters on the Interstate, one of which claims to have a lamp-out bypass (as I gambled earlier on that as the problem/solution).
I've worked on wiring/lighting issues on a few trailers and vehicles in the past, but this situation has me stumped. It's as if the harness/plug isn't doing its job of sending 12V
to the two turn/stop bulbs/filaments when the brake is pressed. I started testing the 7-pin receptacle with a voltmeter, but apparently there's a "smart" system onboard to detect the presence of a trailer - a system that appears to send little or no voltage to the pins when no trailer is attached. This is my first experience with such a "smart" system. Any ideas?