Japan's Mazda Motor Corp. yesterday announced plans to build a $450 million car assembly plant in Flat Rock, Mich., on the site of a former Ford Motor Co. metal castings complex.

Mazda's announcement in Detroit yesterday ends months of speculation about the auto maker's manufacturing intentions in the United States. And it means that Mazda will become the fourth Japanese car company to start production on American soil.

Here are the available details of Mazda's plans as presented yesterday by the company's managing director, Hirota Iida:

*Mazda will build an assembly plant on the former Ford site at Flat Rock. Mazda will raze much of the old castings plant, but leave other portions for yet undetermined future use.

*Mazda will build up to 240,000 cars a year at its new plant, and employ about 3,500 people at that full-production level.

*Production will begin in 1987.

*The United Auto Workers union will have some form of representation in the Mazda plant. But details of the company's "understanding" with the UAW were not presented yesterday.

Three Japanese companies currently build cars or trucks in the United States: Honda Motor Co. Ltd., Nissan Motor Co. Ltd., and Toyota Motor Corp. Of those, only Toyota has established a relationship -- still awaiting final action -- with the UAW.

Toyota and General Motors Corp. will start producing subcompact Nova passenger cars in Fremont, Calif., on Dec. 4 through a joint-venture company called New United Motor Manufacturing Inc.

Honda makes Honda Accord sedans in Marysville, Ohio; and Nissan makes subcompact pickup trucks and subcompact Nissan Sentra passenger cars in Smyrna, Tenn. Iida declined to give specifics on the kind of car Mazda will build at Flat Rock. Auto makers normally are reticent about products in planning. But industry speculation is that the Flat Rock car will be a version of Mazda's subcompact GLC or of its compact 626 passenger cars. Both are front-wheel-drive machines.