Mo. Rev. Stat. § 302.440
In addition to any other provisions of law, a court may require that any person who is found guilty of a first intoxication-related traffic offense, as defined in section 577.001, and a court shall require that any person who is found guilty of a second or subsequent intoxication-related traffic offense, as defined in section 577.001, shall not operate any motor vehicle unless that vehicle is equipped with a functioning, certified ignition interlock device for a period of not less than six months from the date of reinstatement of the person's driver's license. In addition, any court authorized to grant a limited driving privilege under section 302.309 to any person who is found guilty of a second or subsequent intoxication-related traffic offense shall require the use of an ignition interlock device on all vehicles operated by the person as a required condition of the limited driving privilege, except as provided in section 302.441. These requirements shall be in addition to any other provisions of this chapter or chapter 577 requiring installation and maintenance of an ignition interlock device. Any person required to use an ignition interlock device shall comply with such requirement subject to the penalties provided by section 577.599.
§ 302.440, RSMo