On the early cars, there is an onboard display of charging voltage, temperature etc.. It temporarily replaces the trip meter display.
You get into it by holding the trip reset button & turn the key to the first position, then scroll through the options.
It's in the handbook & there's a detailed link here somewhere (that I can't find!).
The battery terminals & grounds all need to be Clean, Bright & Tight.
If only the headlights are affected, there may be a loose plug or wire or a separate ground for them.
