Flickering Lights

2 guesses potentially wide of the mark.... :)

1) is the dash pod earthed, i had weird problems once because it wasn't, indicators not working and dash and dome light diming instead. once bolted back in, no problem.

2) The application of headlights puts load on the battery which in turn results in a load on the alternator. because the regulator sees it as a slight dip in battery voltage due to the lights being on (headlamps are quite power hungry). It keeps alternator ON.... when you switch them off the load disappears a bit, if you have a fully charged battery the regualator will flip-flop the alternator off on off on if the battery charge is on the knife edge between full and not so fully charged. if you have an ammeter in the dash that works, you will see it twicthing in time with the lights.

solid state regulator seems to do this more
i never had the problem with the old mechanical vibrator style

if so its just a function of your new modern battery, how charged the battery is and the way the modern regulator does its switching.
if you swictch on fan and wipers and it stops.... just like it does with headlights....well.

temperature also plays a part, might do it in the summer and not in the winter or vice versa .

i'd guess most are running around with a battery spec, much like this (or bigger).
they fit more into the space these days

Model 096, 278L x 175W x 190H, Wet, 12V, 75Ah, 700A
096 is Euro numbering for a flip top/top up battery that fits the tray with the right connectors and its designed to start a diesel or small truck motor 1970s to 1990s i.e not a battery designed for cars /SUV with 180 amp alternators and ECU charging control.

i would guess the one fitted originaly would be lower specification, installed at the lowest posible unit cost,designed to last the warranty period. On starting the car it will have slumped down below 12 volts and taken longer to get back up to 14 or so volts than anything we fit today.
i.e once car started, the alternator had a bit of work to do and the regulator would switch it ON and keep it ON at any rpm over idle for minutes rather than moments.. The system would not be left trying to make its mind up every 1/2 second. Starting the car would have had an appreciable effect on battery charge which we just don't see with a modern-ish High capcity battery and a denso starter.

really depends if its just a flicker or they are going off and on like your hazard lights...that would be a whole different thing.

Dave