The nation's sixth auto assembly plant involving a Japanese carmaker officially opened for business this week, with the dedication of Diamond-Star Motors Corp., the 50-50 joint venture between Chrysler Corp. and Mitsubishi Motors Corp.

The 2-million-square-foot plant, the centerpiece of a $650-million joint venture between the two carmakers announced more than three years ago, was jointly dedicated by Chrysler Chairman Lee A. Iacocca, Mitsubishi President Toyoo Tate and Illinois Gov. James R. Thompson."We are cutting a ribbon on a whole new era for Chrysler, Mitsubishi, the community, state and maybe a whole era for America as well," Iacocca told an audience of about 2,000 people including plant workers, company executives and state and community officials.

"Two companies, one Japanese and one American, investing in America to do business in America - that's the simple story of this plant," Iacocca said.

The dedication of the plant, about 135 miles southwest of Chicago, marks the end to nearly two years of construction, bringing to reality a program announced by the two carmakers Oct. 7, 1985.

A first shift of about 1,300 workers began building cars last September, about two months ahead of schedule.