water leaking inside thru cowl grate

Compressed air, leaf blower, shop vac at all available opening.
Suck blow, suck blow, blow suck, blow suck, suck suck suck, blow blow blow.
Unless you pull the heater box and can reach up from inside the car.
Not fun.
I'm not above cutting a round hole in the front of the cowl if necessary and then using a body plug there.
Done it on later cars that had one side with a hole in it anyway for some stupid vacuum control thing there.
IF you manage to get it cleaned. spray Rustolem Rusty metal primer all up in there.
Cowls are rust time bombs.