I've read this whole thread. Thought I understood the issue completely.
My boss just upgraded from his '07 gas Silverado 2500 to a '23 Diesel Silverado 2500, and it has been pulling both of his 16' dual axle trailers fine. Until he charged up the big battery that runs the hydraulic pump on the dump trailer. That's when we noticed the "Check Trailer Wiring" message in the DIC. On and off multiple times while driving, and no trailer brakes.
So, I have a diode and was looking into the best place to put it, but first I simply disconnected the line on the big battery which connects the emergency breakaway switch from the brake system, and this did remove the +12 V from the 7-way trailer connector. (on the +12V pin) Seems to me this is the outcome the diode would provide, while leaving the breakaway system intact.
But, even with no current feeding back on the trailer connector +12 V pin, the truck still does not like it, and continues to read "Check Trailer Wiring".
The truck has no problem with the other (non-dump) enclosed trailer by the same manufacturer. Also, his '07 has no problem with either trailer.
I am still not understanding this system.