Here's a picture that I found on Google, showing the underhood fuse center, and the post where the red wire should connect to:
Notes:
- When this is active, the charge requirement is *averaged* between the underhood battery and the battery on the trailer, so if one battery is significantly lower than the other, don't expect the logic to charge the one with lower state of charge (SoC) at the expense of the one with higher SoC .
It's also a fairly low trickle-type charge that goes to the rear trailer battery. If you have dual batteries underhood, or multiple batteries on the trailer, then the charge is averaged between all of them.
If you need better / faster charging for the battery on the trailer, you can get a DC-DC step up charger (one example is
HERE ; there are others. These come in handy if you have a large battery bank on your trailer / RV, and need faster charging than your truck's system will provide by itself. I don't run one (yet), but I've heard nothing but good things about them.
- While this photo references 'brake controller'... if you're going to run one, the better way to hook it up is under the dash. A couple of companies (Tekonsha is one) make a pre-wired harness that plugs right into the fuse box by the driver's side kick panel, making connection of your TBC a snap (mine even terminates at the other end to a plug that goes right into the back of my TBC).
It's dark as I write this, so if you want a picture of mine, you'll have to wait a bit. Or search online 🔦
If you have the owner's manual for the truck, it'll show this connection and detail which fuse goes to your trailering harness.
Finally, the connections can corrode over time at the plug end, so if you've got everything hooked up and still no +12v at the plug, you'll probably want to check continuity at the 7-way connector. You may need to clean up one or more pins (and some dielectric grease helps keep things in check, back there).