Mine eventually went snap after 16 years and so I bent over and took it from Porsche to the tune of £140 for a new one....
Here's where it snapped (where they all snap eventually)
Removal and refitting is pretty straight forward.
After removing the cap over the retaining bolt, a 13mm socket will undo the (slightly rusted on my car) nut.
This will then allow you to prise off the small rubber surround to reveal the larger nut holding the arm in place against the motor.
I used an adjustable wrench on this one, so no idea what size it is.
Once this is undone, the assembly can be removed from the motor arm. This wasn't as easy as I thought it was going to be and took significant upward pressure. I had to be careful not to lever it out against the glass for fear of cracking it. Fortunately there is a groove on one side that allowed me to get one side of some needle nosed pliers in, and I was able to pop the assembly out of the screen. This will make more sense if you ever do this job...
So you're left with this (above) - an opportunity to clean and grease more stuff.
Reassembly is (obviously) the reverse and took no time at all. The wiper now sits in a more appropriate place - much further towards the edge of the rear screen. Not sure if that's because the design has changed slightly, or whether over time, the arm flexes in such a way as to allow the wiper arm to sit further in towards the middle of the screen.