We ran into this problem with our late-2018 MX. The 7-pin socket couldn't power the trailer or its RF brake controller.
Armed with a multimeter, I traced the "missing" 12V aux from the 7-pin socket back to where the towing wiring harness connects to the vehicle. Indeed, the red wire on the side of the wiring harness plug was on the wrong side, just as described in the first post.
The plug itself is a Molex 034985-1606 connector. I used the
drawing to figure out how to pop the front off (the white part, it takes very little force) to gain access to the internal retaining clip for the red wire. Once I got the receptacle terminal for the red wire out, it was just a matter of swapping the red wire and the blue silicone plug and reassembling the connector. I tested with a multimeter again and saw a steady 15.5V on the aux pin, as expected. I reattached the 7-pin socket bracket and everything worked, including the prodigy RF controller.
I didn't have to do any disassembly beyond part of the plug and the two screws holding the 7-pin socket bracket. I needed to move the bracket out of the way so I had enough room under the bumper to fit my hands. The wiring harness itself was attached to the body at its midpoint with a zip-tie, but I didn't end up needing to remove and replace this. There was enough slack in the red wire to swap sides without any further adjustment of the wiring harness.
IIRC, AUX is powered any time the vehicle is "on", even in P.
Good luck! A multimeter is your best friend for this one, but it ended up being a simple repair.