Nissan became the third Japanese auto maker to build passenger cars in the United States when its first American-made Sentra was driven off the assembly line at its plant in Smyrna, Tenn. The $745-million plant, equipped with 228 robots, opened in June, 1983, to build light pickup trucks. Since then, more than 150,000 trucks have been assembled. Pickups were built first because the U.S. charges a 25% tariff on imported trucks, compared to a 2.8% tariff on cars. Nissan joins Honda and Toyota in assembling autos in this country.