For the Volkswagen models listed here and their corresponding model year, the wiper shafts are sealed at the top by bearing covers.
If the bearing cover is cracked or missing, moisture will seep in between the shaft and the outer casing, causing the assembly to run rough over time.
If you're a bit unlucky, the wipers will run so much that the wiper motor will fail, all thanks to that little plastic cover.
If you remove the wiper arms from the shaft, you'll see the cover, if not, it's advisable to install it to preserve your wiper system.
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