This is only on Lariat and above with key fob instead of a traditional key.

 

I think the confusion here is the tailgate on a higher trims does not actually "Lock" in the traditional sense.

The button on the tailgate activates a solenoid that opens the tailgate latches. When you lock the doors the computer ignores the button on the tailgate so it won't activate the solenoid.

Now if the key fob is near the tailgate it will allow the solenoid to work so pushing the button opens the tailgate.

With this in mind you will not be able to use the wiring in the tailgate to lock your cover. You will have to try and tap into the door locks.