The plastic cowl under the windshield wipers covers a type of tunnel area which houses the wiper motor and linkages and also the air intake to the cabin blower motor. This area is sealed against rodents and debris EXCEPT for vertical openings on the ends (between the fender and the ends of this channel). Plug these holes and you will stop rodents and garbage from going through the blower and plugging the cabin air filters. We had squirrels/chipmunks dragging anything/everything in through this opening on a 2002 Sierra 2500HD. NOTE: leave small holes for drainage of water. Eventually these small holes may plug with accumulated muck and you will have to remove your plug and clean out and reinsert the plug. A better fix would be to fasten a screen ontop of the blower air intake that is in this tunnel. You can access this intake by prying off a panel on passenger side of this tunnel that is factory sealed with caulk and painted over. (Not obvious that it is a removable panel)
