[2manyT/A'S]

The rust in the cowl/dash corners is an internal thing...

The roof has an inner sheet metal frame/ liner......With temperature changes...the inside of the roof sweats...just like the underside of the deck lid.....

This sweat/moisture runs down the inside of the windshield pillar causing the rust you see in the windshield lower corners....one side only rust...means car was parked on an angle
If it was stored outside and regularly driven....the heater would take care of the moisture...and reduce the incidence of rust....

The only way water could get past those plastic deflectors would be if car was driven at highway speeds...as it would need to be driven uphill...