i have a car with these in the rear and a car with them all around.
i rarely have to use more than the hose to get every single thing out. i just flood the heck out of them and put the sprayer on the hardest stream and hit them hard in an organized pattern. on rare occasion i will spray some Power Clean and agitate a bit with a Daytona.
my wife ran over an exploded container of some sort of industrial expanding foam. it got all over the place including all four fabric wheel wells. a power washer obliterated every single piece of that junk. so, that is my last stop if the regular hose doesn't do anything.