Good news, it works ! The new harness and the new installation instructions worked, with a slight modification. Turns out the kit gets its power from the trailer battery circuit, the red/green wire in the trailer connector. The fuse for that is #63 (30amp) on the fuse panel under the hood. Gen5 told me that sometimes this fuse might not even be installed, so I checked it and found it was blown. In the new instructions there are two ways to connect the 3 wires (2 from the harness and 1 from the trailer tap) that connect to the switch.
Depending on how your truck was wired, you connect those 3 wires to the switch differently. Pic 3 and Pic 4 show that in my truck the Black wire matches up on the truck wiring and the harness wiring. But I had to use the instructions for the opposite setup to make it work. I think those directions on the Gen5 site might be mixed up, as I connected it as directed and blew the fuse. Connected it using alternate instructions and it works just fine. Not a big deal, clearly a new product.
Also you don't have to drill a hole in the bed for the switch. There is a rubber plug on the side wall by the tailgate on both sides. I think it's for one of those fold out handles or maybe a tool box accessory. Pic1. I found that removing the clips on the supplied switch, allows you to insert it into that hole with no drilling at all. (pic 2) I popped the tail light to make it easier to run the wires. Pic 5. Just pop the LED bed light out, (press up under the bottom edge with a plastic tool, there's a compression clip, pop whole fixture up and out. Then you can fish the wire up into that hole, and up to the small hole that you can run those wires up the bigger hole.. You will need to extend the red power wire a few more inches to reach the trailer harness, but well worth doing so. I also think that the switch mounted high like this will help it from getting crunched by cargo.
All in all a great and easy to install feature. 30 minute install time. Now I can open the tailgate and hit the switch to light things up without having to go back to the cab or even opening the door. Remember that switch will have power all the time, so if you leave that switch on, you could potentially drain your battery. This setup only controls the bed lights. but not the overhead light on the cab or the task lights which would have really nice, but I'm happy just being able to turn the bed lights on, as most of the time the cover is on anyways. The switch does feel a little cheap, but it seems like most of aftermarket automotive switches are that way, the rubber seal seems pretty good though.
All in all - well worth the effort, easy install. The fact the switch fits into that ready to use hole is a huge plus. Great support from the Gen5diy team.