I'll try and take some pictures tomorrow when I make absolutely sure I have got all of the rubbish out of the one below the wiper assy. I used a strip of rigid wire to poke the drain the other day but could probably get more gunk out with a steel ruler or such like.
It was a combination of soil and grit that I extracted from the drain below the wiper.
I'm sure the cleaning out of this drain was added to the B service schedule some time ago - my car has had around 10 B services at MB dealerships.