169.48 VEHICLE LIGHTING. Subdivision 1.Lights to be displayed. (a) Every vehicle upon a highway within this state: (1) at any time from sunset to sunrise; (2) at any time when it is raining, snowing, sleeting, or hailing; and (3) at any other time when visibility is impaired by weather, smoke, fog or other conditions... The generality of it. It is not directly about wipers and headlights, but it is a good rule of thumb re. (a)(2). That point (a)(2) isn't necessarily about you being able to see, it is as much or more making your vehicle more visible to others.