Wis. Stat. § 345.06: Owner’s liability for act of operator.

The owners of every vehicle operating upon any highway for the conveyance of passengers for hire are jointly and severally liable to the party injured for all injuries and damage done by any person in the employment of such owners as an operator, while operating such vehicle, whether the act occasioning such injuries or damage was intentional, negligent or otherwise, in the same manner as such operator would be liable.