I feel like automatic headlights should be mandatory. In my state, if your wipers are on, your lights should be on. Even during the day rain can make it difficult to see cars that don't have their headlights on and therefore don't have red lights on in the rear. I see many people driving around at night with no lights on and it's a safety hazard. Additionally, your dash lights will be at full brightness when the headlights are off so now your eyes can see even less light coming through the windshield. If relatively expensive backup camera and a display to show it can be mandated, then a much cheaper photodiode to switch on some lights should definitely mandatory.

I wonder if the insurance lobby was the one that pushed for backup cameras, it's probably greatly reduced the number of rear collisions that they have to pay out.

I noticed that when there's heavy rain, silver cars are practically invisible in rear view mirrors unless their lights are on. Particularly when there's plenty of water on the rear windshield.

I think in my developing country daytime running lights have been made mandatory for all new vehicles, but the millions of existing vehicles don't have them, and their drivers don't bother to turn them on during rains.