Tailgate?

[isis]

I would imagine a tailgate has drain holes designed to function when the gate is closed. It’s basically a door.

 

[MRV99]

Dude that is the pressure relief regulation port. Since the truck is aluminum, gas build up due to a reaction. If that gets plugged it could cause the tailgate to swell and it will not open/close. Check your doors, you have similar relief holes at the bottom. They do not cover it under warranty if they can prove you plugged it. True story

 

[FINRPTR]

I used it to open the tailgate when the electric motor stopped working. Had to make a tool to get in there to access the mechanical part of the release assembly. The tool looks like a distributor wrench. I keep it in the truck in case it ever fails again.