thats bad,
this is due to neglect of never cleaning this place and never refinishing the cavity wax that was applied here from factory. Its a regular maintenance procedure done every 4 years or so.
This corrosion is accelerated if you use very caustic soaps to wash the car too often, too caustic soaps are like clothing and dish washing detergents, surf, lemon max, ajax etc etc.
To repair this, the car will need to have a few pieces removed, from the outside it will be the wiper tray, wipers, all associated wiring and sometimes the mudguards too,
from the inside you need to remove the complete dashboard, the wiring harnesses behind it, the insulation padding, etc to ensure the car doesnt set on fire due to welding.
you will need to tear this apart some more to get an idea of how bad the corrosion is. Also set about finding a can of cavity wax spray. That spray is done as the very end job after this area has been welded and primed or painted. The spray wax is then applied heavily - its very slick and prevents water from pooling.
When the job is done, make sure the area is draining properly - meaning, make sure the denter ustaad doesnt make a deep end here where water can pool or try to weld the air intake port shut because "ac acha chalega". You need that fresh air vent for the cabin, there are two exhaust vents at the back of the car too.
