This should be done annually. I was even following a thread on the forum about it and this is a common issue. Mercedes wants this done annually I believe. They aren't kidding.
Unfortunately I did this last at the end of everything else and didn't take any pictures. It would be easy to take them after the fact just by raising the hood.
Basically the two inner drains exit with a round tube about an inch in diameter. The pass side one is an HVAC intake air box drain. The driver's side one is basically just where water goes that flows into the matching other cowl grill on the hood. Remember this car is sold everywhere in the world so they just reverse this for RHD. Anyway, these drains were pretty much fine. The airbox needed a tiny bit of flushing and shoving a fish tape in there once to break up the clog. Easy.
The real issue is the outer drains, which seem to be the ONLY place windshield water is supposed to go. And both the right and left ones were totally clogged. Awesome.
I spent a long time on this trying a variety of techniques. On the driver's side, what seemed to work was the fish tape, and flushing with a garden house (but be careful, the front SAM is right there).
On the passenger side I wasn't as lucky. What ended up working there is I found a length of clear, vinyl hose. I shoved the hose into the downspout drain to help force water in and held the other end to a garden sprayer. I would pressurize the drain but that still didn't work. It was backing up. So after loading it with water, I quickly would hold everything steady and give the hose a shot of shop air. This forced a water slug through the hose. I did this a couple times and eventually it blew the clog out! I was then able to flush with a garden hose.
The key thing is, if they can't continuously take a garden house on full jet spray, they are clogged. Going forward maybe this won't be a problem if I just add it to my oil change process. I'll report back in 5,000 miles.