Here's pics of my fuse box and the 7-pin connector under the rear bumper.
If you look in the top left corner of the fuse box, there should be a red dummy fuse in the spot marked "stud #1" if a previous owner hasn't removed it. When you take the dummy fuse out, you'll realize it's made out of all plastic, no metal components and of no use other than to fill a fuse spot. Take that out and replace with a 40 or 50 amp square fuse in it's place. There were two different types of square fuses at the auto parts store, so you'll want to look at the small tag that came with your trailer brake controller; if you got the harness designed for the H2, its tag will have instructions on what fuse type you need and what size (I didn't realize I needed to look at the tag until I was done guessing--it would have saved alot of time!). I guessed at the time and got a 60 amp fuse, which is too high of a rating but works.
To be double sure you've got power, go to the 7-pin plug under your rear bumper and put a meter or light on the top right socket, it's marked with a "+" symbol on the little door that covers the plug-in connection; this socket should have power regardless of the truck being powered on or not.
Fuse box lid indicating "stud #1"
Note 60A fuse in top left corner, that's the one you want to put a 40A or 50A fuse in
Check the top right socket marked with the "+" symbol for power--it should be hot regardless of vehicle being on or not
