As mentioned the wiring harness from Ford is in the left frame rail. It stops at the end of the Ford supplied frame. If you have frame extensions open the wiring harness where the original frame and the extensions meet.
While the blue wire in the diagram shows a #12 wire the one in our 2001 F53 chassis was a #10. It is folded back and taped inside the harness. If the body builder didn't disturb it there should be a tag on it labeled "trailer brake".
The difference in wire size could have been a running change during the model year. A #12 wire is adequate for a trailer with a single axle, but a #10 is recommended for anything larger.
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2013 Adventurer 32H
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