Yes, a repo man can enter private property in Wisconsin to repossess a vehicle. They cannot enter a locked garage or closed space, or even unlatch a closed gate to do the repossession.
It is important to note that even if a repo man has permission to enter your property, they must still follow the law. For example, they cannot use excessive force to repossess your vehicle, or breach the peace. If you believe that a repo man has violated your rights, you should speak with an attorney.