Brushes on the voltage regulator, is my stab at it.
(held in by 2 screw on the back of the alternator - I'n guessing the brushes might be little nubs)
That, or maybe a bad ground...but I'd check the brushes first.

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D