It's all installed!
I got the side piece and the white pieces easy enough. But I really couldn't get the 8 switch panel out easily. I could see the 4 side tabs, but I had a top tab also. Once that click it came out quickly.
It was also tricky to get the switch plug out. You don't push on those little triangular tabs like you would expect, you just slightly pry up on the part they clip into.
Then I had to nibble out the interior of my switch. This was pretty easy but it bulged out the plug. I didn't like that I couldn't firmly attach it.
Maybe you can see the bulge here, I'm not sure.
I went to dry fit in the car and I couldn't insert it! The bulge was preventing it. But, I inserted it empty into the slot and then the remote fit right in snugly.
It was just then a matter of physically mounting the box, electrically connecting everything, and the zip tying the wires out of the way and putting the trim pieces back.
Over all I'm 75% happy with it. You can turn the button and push the button. But the only physical connection is a ad hoc press fit. I can see a failure mode where when you go to press the button and it pushes into the dash.
I also don't like the rat's nest of wires under foot. Sure, I zip tied them all up, but it isn't elegant. I wish Subaru would offer a factory brake controller.
And thank
everybody for the help! We get the trailer this Friday!