The wiring loom connecting, among other things, the alternator to the voltage regulator, is located in the engine compartment and is surrounded by rubber grommets.
This is the ideal place for the rubber to harden due to the high temperatures and gradually crumble. No matter how carefully you maintain the rubber, you can prolong the aging process, but you can't stop it.
So there are very few Volkswagens with original seals that do not show cracks. The same goes for the wiring harness, replace them when they start to show cracks, only then will you be sure to have a good seal between the wiring loom, the body and the alternator.
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