You can see the spot welds where the rust is in those pictures
Once you take out the plastic wheel well liners , you will see the inner rear quarter support that is spot welded to the bedside on both sides
Spray inside that support from the backside thru the open spaces with fluid film and plastic wands , spray the hell out of it between the support structure and bedside
It will drip down and coat the areas you can’t reach
Helps to remove the rear tires and jack up the frame
You will see once everything is removed
Start there first
My 2011 is spotless and I live in WI