Use a shop vacuum with a nozzle taped on the hose and keep sticking it down the side. Pull it up and pull the leaves and debris off the nozzle. Repeat till you don't get junk stuck on it. Then use a stiff wire down the side to break up sediment that's cemented together and repeat the vacuuming process.On top. I hate to think how much trash has gotten thru that plastic grill and is on the inside. How hard is that to clean?
If the car is driven most of the water rolls out and the remainder evaporates. It is really only a problem if the car routinely sits wet or gets parked for extended periods wet. . The depression above the brake pedal provides clearance for the reciprocating transmission for the windshield wipers so you have to be careful not to fill it till there's interference. On the other side, the little pond that can form a few inches to the right of the passenger windshield wiper mount only does so if the car is parked sideways on a hill with the passenger side lower, rarely is that one rusted through. But no fear, a leaking heater core sometime during the cars life usually rusted the passenger foot well and toe board anyway. Between cowl rust through on the drivers side and core leaks on the passengers front floor repairs are needed very often on 2nd gens.That's crazy if GM didn't provide a drain hole and a tube for water to run.