Legal Question in Traffic Law in California

I assume you mean that the entire light is physically missing and not just burned out or not working. I would think it is illegal as you are not able to drive a car that is not in proper functioning fashion, whether or not you need that function. If your windshield wipers do not work, do you l think you could claim the car is driveable because you would not drive it while it is raining? How do the police know that would be true. How do they know you will not drive at night or through a dark tunnel, etc.

Not illegal unless it's raining during the day and your wipers are on.