I have a 02 GMC 1/2 ton.
Took my chrome wheel well trim off to check for dirt-rust.
Found rust coming from the inside top wheel well, packed with old dirt, not the inside lip but in a space that you can only reach from inside the well, from the front or rear under the truck. Cleaned and put stop stop-rust stuff and then sprayed undercoating on it. There are small plastic plugs you can remove to help clean. Dumb design!