Wis. Stat. § 347.42: Windshield wipers.

No person may operate on a highway any motor vehicle equipped with a windshield, except a moped or motorcycle, unless the motor vehicle also is equipped with a device for cleaning rain, snow or other moisture from the windshield. The device shall be so constructed as to be controlled or operated by the operator of the vehicle and shall at all times be maintained in good working order.