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What you'll want to do is pull the fuse box up and check for power on that wire right where it comes out of the fuse box. If there is no power right at the bottom of the fuse box then you know the problem is internal in the box.
All connections are made inside the fuse box by printed circuit board, no wires are inside. Moisture can cause corrosion spots on the circuit board which will cause open circuits. you could have an open circuit between the fuse and the trailer tow right turn/brake relay, an open circuit from the front control module do the relay, or from the relay's output to the connector at the bottom of the fuse box. These relays are permanently soldered to the circuit board inside the box, which is why you can't find it.
If the problem ends up being in the box you can replace it, or you may be able to fix it yourself. I've actually had good luck opening them up, finding and repairing the problem. HERE is a video I made showing the process.