Mont. Code § 25-3-603
The operation by any person, individually or by the person's agent, of any motor vehicle, whether registered or unregistered and with or without a license to operate, on any public way in this state is considered equivalent to an appointment by the person of the secretary of state to be the person's true and lawful attorney upon whom may be served all lawful processes in any action or proceeding against the person growing out of an accident or collision in which the person or the person's agent may be involved while operating a motor vehicle on any public way in this state. The operation is a signification of an agreement by the person that any process against the person that is served upon the secretary of state is of the same force and validity as if served upon the person personally. This section does not apply in case of any cause of action, for the service of process in which provision is made by 25-3-602, and it does not authorize service of process upon any person who may with due diligence be found and personally served with process within the state of Montana.
§ 25-3-603, MCA