Got a cover for my tire carrier? No directions. Am I suppose to take stock off? Or leave it on. With stick one off it will allow water to get in past the mesh screen part of the new cover. With it on of doesn't fit normal...

 

Remove the stock piece and bolt up the new cover. Just that easy.

 

That part is inadequately engineered IMO. The surface area of the new vent is much smaller than the stock exhauster, and it's clearly not water tight. Easy solution though would be to mount it with longer bolts and 1" bushings to stand it off the tail gate, leaving the stock exhauster in place and operating as designed.

Edit: in case anyone is wondering what that part is for, it has one main function that comes into play in various circumstances.

It allows air to escape when you shut the doors. Closing the doors creates a momentary high pressure situation. Without the vent, the doors can be more difficult to close.

It allows air to escape when you're running the climate control system on fresh air. If you bring air in, it needs to go out. Otherwise you would just pressurize the cabin, if you have a soft top it would balloon like crazy. Eventually, increased cabin pressure would restrict incoming airflow and diminish the effectiveness of the climate control as the fan would struggle to pump air in.

It allows air to escape during an air bag deployment. Air bags displace a LOT of air in an instant. That air needs somewhere to go. The exhauster gives it an exit.