Groover,
Well Sir, you'd have some oddly acting trailer brakes if you managed to tie into the "Diagnostic" front port on your rig. Yes Sir, that plug you found is for engine/chassis diagnostics for techs to use.
Most Winnes do have a trailer plug pre-wired for a controller under the dash, on the left side of the column. But, it's not "mounted". It's usually hanging, tied up with a wire tire to one of the zillion wiring bundles under there. And, a few inches from the plug itself will be a tag taped to it. It should say, "trailer brake connection".
It is possible that that plug, has drifted into the front, lower compartment too. That is of course if you have a front left compartment. On mine, it contains many fuses, circuit breakers, MMDC computer, Smart Wheel master controller, Vehicle Security Module and much more. And, obviously some of those wires go up into the dash area via a passage way and, it's possible that that trailer plug has slipped down into that compartment. Again, that's only if you have one. Good luck.
Scott
