I verified that on my 2018 MBS, the terminated blue wire under the seat is connected to the 11:00 o'clock blade of 7-way trailer recepticle. That is, looking at the trailer-side plug from the recepticle side, with the external bump guide down, it is the blade in the upper right quadrant. The standard wiring calls for that to be +12vdc from tow-vehicle-installed controller to activate electric brakes on trailer. Note that the adjacent blade in the upper left quadrant is ground. I have no such controller and am now wiring my trailer harness to feed the trailer brake indicator light from my Duo-Stay-In-Play brake installed in tow vehicle back to led indicator lamp I am installing in dash below radio.
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