Toyota Motor Manufacturing North America Inc., a consolidation of the automakers' assembly, accounting, finance, planning, purchasing, quality assurance, legal and supplier-support operations will open this fall in a yet-to-be-purchased site in northern Kentucky, just outside Cincinnati. It will start with about 350 employees, most of whom will transfer from other Toyota locations. The center, which will be about 60 miles north of Toyota's Georgetown, KY, assembly plant, is expected to employ about 560 by 1998.