Open the underhood fuse box and there'll be a series of studs on the battery side of the panel.
There will (likely) be one that has no wire on it - it is for the upbuilder to install the B+ Battery power to the trailer 7-pin connex under the driver's side of the rear bumper for trailer lights and brake controller and battery charging circuit.
In the real world - that means: where they actually enforce the DOT standards, there is SUPPOSED to be a 12V battery on the trailer if it has electric brakes and I think the weight lower limit is around 1500 lbs.
The whole idea is that if the trailer comes off the hitch, there is a rip-away lanyard that activates the trailer brakes from the power in that battery to stop the trailer eventually before it kills a busload of nuns on the way to a wine tasting party.
Anyway - this is a good place to put your 12V tap for an auxiliary circuit. Let me see if I can find one of my pictures of that lig --- BRB .... (OK - don't want to misrepresent the authenticity of the producer of this photo - but mine is lost somewhere in the interwebs, so this is one I found - er: somewhere)
Look at the small stud in the top left corner. This is the one you want.
The correct fuse is already installed right next to it - but it may not be in your fuse panel. Just add it and the power will go to the 7-pin and you can add another rind-crimp connex there for your own additional purposes - up to the amperage limit of the circuit that is.
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