Electrical faults are often hard to find, with grounding problems usually being the root of all evil.
Make sure that all grounding points are free of oxidation and that the mounting locations make good contact.
On our VW models, the earth is obtained directly from the battery terminal, which is attached to the chassis by an earth cable.
Check the condition of the earth cable regularly, and replace it if it becomes oxidized or damaged.
The earth cable shown here is of a similar design to the original, so it also features an additional flat connection, which is missing from many reproductions.
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