After September 30, 1991, or upon occurrence of a vacancy in either the office of tax assessor or tax collector, there shall be a county revenue commissioner in St. Clair County. A commissioner shall be elected at the general election in 1990 and at the general election every six years thereafter, who shall serve for a term of six years beginning on the first day of October next after his or her election, and until his or her successor is elected and has qualified. (Act 88–316, p. 478, § 1.)