I was bored and I've never gotten around to doing this, so I thought I might as well. Apparently the wiper mechs on Mercs with single blades can dry up over time, which can lead to corrosion and eventual seizure. You can imagine what Mercedes would charge for a replacement!
Someone had already sprayed the outside with copper grease, not a job I'd recommend as it is a grinding paste on parts like this, so I cleaned most of it off with the paper towel.
Here is the procedure, using my car:
Tools required - waterproof grease, silicone spray, paper towels/rag, long screwdriver with flat blade
1) Turn the ignition to position 2 and activate the wiper. Turn the ignition off when the wiper is in the 12 o'clock position (will take a few goes but its not hard).
2) Remove the keys and release the bonnet catch. Open the bonnet and depress the catch in the corner so it opens to its full vertical position. This will give you more room to work.
3) There are 2 bits of plastic that cover the mechanism. The first one is the smallest section, its easy to release. I forgot to take a picture but basically get your fingers between it and the glass of the windscreen, and tug both sides outwards. If your fingers are about half way up the bit of plastic, thats where the clips are. They're not big clips and it comes off easily. Once its free, it will rotate upwards around a pivot at the bottom of the wiper, just pull it up until it won't go further, and then pull it away from the windscreen (check the picture as I'm re-assembling it at the bottom of this page).
4) The larger piece of plastic is tricky. The first 2 clips are similar to the smaller bit of plastic. Get your fingers on one side of it, similar to above, and give it a couple of small tugs. You should easily find the clips, again they're only small.
5) Once you've found these clips, you'll notice that the big piece of plastic is still held on somehow to the wiper.
edit: I've since been told that the correct procedure is to remove the hex bolt and the plastic cover that way - so please ignore this step unless strictly necessary! I've put the original (and apparently incorrect) method in italics.
Now I got a bit of an old car mirror and had a look around, but I couldn't see any other clips. So what I did was get my screwdriver, poke it through the top (screwdriver aimed towards the glass), and push outwards. You can see in the picture below how the clip is shaped - its almost circular, and you have to give it some force to get it off. Fear not though, as its fairly flexible plastic and if you're careful it will come away easily enough. Careful not to break the glass!
http://static.flickr.com/73/204066577_c4a154425a.jpg
You can see the circular clip - there is a break at the bottom. I have no idea if thats how the part is manufactured, or if mine had been broken previously.
6) Now both bits of plastic are off, you can see the mechanism.
Mine still had a bit of grease on, so either I've been lucky, they've used a miracle grease, or someone has serviced it at some point. Still, it could have done with more, so I obliged with Teflon grease (only grease I've got):
Once you've greased it up good, I gave it a bit of a spray with silicone lubricant to get to parts I couldn't reach. Once I'd done that, I reassembled everything in reverse order to above, and Bob's your uncle - one nicely lubricated wiper mechanism!
