Trailer Disconnected alarm

I see people have posted about this for nearly 10 years but no one has really found a smoking gun.

I now have to do something. The alarm might stay quiet for 20 miles but then it will go off ten times in a minute. A chime, "trailer disconnected" and then in a couple seconds another chime and "trailer connected". Of course nothing is actually disconnected. The brakes and all the lights are working even when it says "disconnected".

I've done the normal things people do. Check the fuses. Clean the plugs with contact cleaner and plug together really tight. Disassemble the plugs looking for loose wires etc. Visual inspection of all trailer wires end to end. Had my local dealer put a testing device on the trailer plug for diagnostics. Came out clean.

If someone knows the fix, gimme. Otherwise, does anyone know for sure exactly what the truck is looking at when it decides the trailer is disconnected? I'm at the point of actually disconnecting from the truck and running my own brake controller (instead of the built-in Ford controller) and trailer light wiring. But if I knew what the alarm is looking at, maybe I could jury-rig something and lie to it.