Fiesta 12 plate leak in rear boot

[Foxyuk]

I have the famous leak it was  diagnosed it’s the rear bumper vent at drivers side below light cluster.  Already had bumper off and re sealed it.   Still made no difference   Has anyone actually taken it out and filled in the hole with expanding foam or something different ie welded a plate 

 

what’s the purpose of it anyway ? 

ready  to sell car and would rather get it sorted once and for all   

[TomsFocus]

The vents are there so that the windows don't explode from the pressure of slamming a door...  I probably wouldn't block those up.  

They also allow full airflow through the vehicle.  If there was no rear 'escape' for stale air, fresh air would not be effectively drawn through the cabin.

If you're selling the car, I think I'd just leave it to the next person to decide how they want to sort it.