I ran into same thing which is why started digging in to the technical details. Hate to say it but it likely is not actually doing anything.
I will post details on TRM to see if better signal there although I suspect the input will be same as that wire under seat. Most likely have to use an output from TRM
I can understand how pressing the gain switch and feeling the trailer brake does not prove that the brake controller is working when apply the brake pedal. I get that.
What I don't get is I can set the gain to very low levels, brake medium hard, then do the same thing again with the gain set at progressively higher levels, and I can feel the trailer taking progressively more of the braking load. That seems to me to mean that the controller is in fact sending a signal to the trailer under braking alone.
I wonder if what we are missing out on is the "bonus" features of the Redarc controller. It is supposed to be smarter than a regular controller using its built-in accelerometer to measure vehicle motion and adjusting its brake signal accordingly. Maybe all we are getting is regular dumb controls? I don't know, I'm just guessing out loud. Either way though, I agree this is something that we should get to the bottom of.