Interesting problem, but at least it is resolved
I am working on a similar issue, but with the gizmo that auto-dims on top of the dashboard on my 64 Rivi.
I get all 4 lights come on, when I hit dimmer, just parking lights go on or off. No effect on headlamps.
Auto dim gizmo is supposed to detect lights and dim, but it doesn't do anything, not even in the override position.
So far, no joy...
I suspected the dimmer switch, but cannot find the one with the extra wire for auto-dimmer, only these ones at Rock Auto
https://www.rockauto.com/en/catalog/buick,1964,riviera,7.0l+425cid+v8,1319475,electrical-switch+&+relay,dimmer+switch,4340
Bought it last fall, did all suspension, engine work, axle, brakes, hoses & lines, converted to HEI, etc., over winter.
Getting ready for safety, need lights to do their thing
Video of my car when I first bought it. Needed some work.