Got around to finally doing my tailgate lock in between putting a new clutch in the Camaro. Wanted to say thanks to the OP and the guy above for pointing out the two wires to put the lock on second FOB press. That's the way it should be, I say. The unlock switches on the doors operate it, two presses of the remote unlock operate it, and one press of lock on all operate it.
I had the wiring done in 10 minutes, it took me a lot longer to prep and run the wire up the chassis. I chose to run my wire inside flex tubing wrapped with electrical tape, similar to factory style. Being up in the North East, anything you can do to keep the salt off stuff is worth it. I ran along the factory harness until I hit the floor grommet and went that route. It's far easier than anything else I saw under there. I re-zipped the wire bundle like it had from the factory. Run the ziptie from the inside under, and when you see the tip on the other side, use small needlenose to pull up on it while you push, you'll get it through. Also I didn't have my good multimeter handy, so I used a cheap one. It was too slow to read 12 volts I think, I saw 5-6v on unlock, and somewhere around -3v on lock. It was a solid connection and operates the tailgate as it should. Tested it at the bundle, at the weather connection, and at the tailgate.
I have a full Line-X and bed cover, and I can hear the lock, but it's not disturbing.
This is on a 2015 Silverado LTZ Crew Cab Regular Bed. (Z71+Backup camera if that matters..don't think it does.)
PS: For a truck with 2,000 miles, it sure does have a disturbing amount of rust already.