Severe voltmeter needle oscillations

Dana67Dart said
What car and year are you working on?
What modifications to the electrical have you or a previous owner done?
as asked before did everything work correctly before?

It's my 71 Demon, /6 198ci, 3 speed.
My dashboard harness is unmolested and strictly stock.
Everything works correctly as long as the engine is shut off

About the ammeter/voltmeter conversion, I carefully followed the MAD mod (Catalog)

I modified the bulkhead connector with big wires passing through.

Then soldered them together

@67Dart273 : Just know that my alternator is a "square back", so not original for a 1971 Chrysler car. I know nothing about my regulator.

Thanks for your help
François