cowl panel

Like Uselesss said you can go at it from the under side of the dash with a shop vac and some kind of attachment. Use compressed air blown through the cowl vent to move any leaves or dirt around. I also read here somewhere that a member cut 2 holes in the front of the cowl(inside engine compartment) and then put some plastic body plugs in the holes when done.
I cleaned mine out when I had the fenders off and could access the sides of the cowl.(there are small drain openings that get clogged and start the whole rust issue). Good luck, hope this helps.