There was a thread about a year ago on installing a Redarc brake controller with instructions attached as a word document. I used it and added an improvement I discovered. The procedure had you cut into a harness in the drivers side rear door opening to get a brake signal wire. In hindsight I think you could also tap into the wire at the rear light fixture on either side. (I did that for a third breaklight on a camper shell)
Like many here, I was really disappointed to learn the plug under the dash was no longer there. The 4th gen truck has a totally different harness to go with the LED lights and bells and whistles that are now standard. But Toyota did us a mis-service making it hard like it is.