There is a panel foam in the dash area where you are referencing to minimize harmonic drumming.
Once the integrity of the seal in is breached and A PINHOLE forms the water is held in place by the foam and then the rot will progress behind the scenes... and that scene has about 4 layers of metal in there...
The firewall, the cowl wiper recess bowl area, the upper cowl Wiper support (where the VIN is stamped), and FINALLY the dash TOP.
the foam is also between the outer wheelhouses and the quarter panels for the same purpose... cars restored without the foam or without something in place of it may experience drumming of the panels.