1985 300SD.

The issue:
At night with the headlights on; there was a distinct/annoying brightness fluctuation that went faster/slower with engine RPM.

I pulled the alternator to replace the voltage regulator.

SURPRISE:
I found the noise suppression condenser melted, drooling it's plastic coating into the alternator..
Drooled plastic interfered with the tin finger that contacts the regulator..

The repair:
* Removed melted plastic.
* Replaced the noise suppression condenser.
* Replaced the voltage regulator.

The issue is gone..