mauro, I remember that when I was going through the posts on water drains, this place was described by one of the Members as leading to cavity that may, eventually, collect water that will spill over to inside of the cockpit. If I were in your shoes, I would blow some air into that drain, see what happens, try to dry it , put some cavity wax inside and plug it (that is what i did in my car). I think the general conclusion on this spot was that it better be plugged. Someone, actually, reported finding remains of some plug there. Check me by search, but I think that is what was said.
I have also other suspects for the place you are describing (other than antenna, which was mentioned):
- on some cars you may have some rust and the "basin" under fresh air inlet may not be water tight; some sealing there also may be missing
- you have a hole for antenna and radio power wires there in the fire wall, closer to the middle of the car
- there may be a hole for carpet holder that is open
- the wiper motor rubber plate may net be sealing the opening as it should
- wiper shafts