[HO455]

Originally Posted by mkmc69

Almost makes you wonder if you should drill drain holes in the lowest part of that vent well.

GM provided drainage for the lower piece. It was designed to direct air through the vent. It doesn't cover the entire area. Water will flow past it (Unless it is full of leaves and dirt) and not fill the cab with water when it rains. (Photo 1 blue arrow) The water flows out at the bottom edge of the fender through some factory provided gaps (Photo 2 red circle). So part of the process to clean the lower cowl areas needs to include blowing air up from the bottom of the truck through the water drains. (Red circle) An air nozzle with a thin long tip works well for this. (Photo 3)
Basically you blow up from the bottom after you have removed as much debris as you can with your hand, (Kids can be a big help with that.) then vacuum from the inside, then blow down from inside and then blow back upwards from the bottom. Do it over and over until no more debris remains. A flashlight and mirror can help with the cleaning. Both sides will need to be done. When I did my Burban it was a 3 hour job before I was satisfied they were clean.
Unbolting the lower part of the fender can help as the entire area under the cowl vent box and the fender can be full of debris. This will allow you to pull it open some and get all of the debris out.