Easy fix this one,
Remove the black plastic panel below the windscreen - it is held in by a number of selftapping torx screws (I think you have to remove the wiper arms as well), now remove the motor from the wiper mechanism (If i remember rightly you have to remove the whol mech from the car - it's held in by a couple of bolts). If you strip the covers from the motor, you should be able to access the gearbox section of the motor, in here you'll see a large drive gear that has a number of metal slip rings attached, these are likely to either be covered in grease from the gears or blackened from arcing. Clean up the rings as best you can, remove any arcing marks with a bit of emery cloth or wet and dry, then lubricate the rings with some contact grease (use sparingly). You should also find some contacts inside the gearbox cover, these are also likely to be arced or covered in grease, again clean, lube with contact grease, and then reassemble the whole lot...
Hopefully, when you test the wipers they should now work 10 times better