All of my VW's, which have been late-production air-cooled (REAL) Beetles, ca. 1973-74, have had this problem. Or non-problem, depending on your point of view. The charging system is electrically identical, as far as I can tell. The physical arrangement is different, but the parts involved are the same.
I saw this with both Bosch and Motorola alternators that were fitted to different cars and functionally identical.
I always chalked it up to poor (i.e. high-resistance) grounds on the instrument cluster, the engine, and the battery cables themselves. These units never failed to charge (except for one regulator failure in almost 15 yrs of use). They always lit the turnsignal light just a bit, and maybe the alternator light dimly as well. Like yours it would change or go out when the headlights were used, and fade back on a bit later. I always found 13.5 V or so when I checked system voltage... not strong, for sure, but I was able to verify with an ammeter that it was in fact charging, and after all it didn't take that much juice to turn these cars over. Nice clean battery terminals seemed to reduce the brightness of the light, but not turn it off. It did seem brighter when the lights were on, but hard to say... and now my last VW is a '70 Bus with a generator, so that's totally different.
I would say, if you get consistently good system voltage, and the connection points throughout the system are clean and tight, enjoy the green glow!
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Rob Bareiss, New London CT ::: "Roterande Fläkt Och Drivremmar!"