Nissan Diesel Motor Co., a major truck and bus maker, posted midterm losses Friday for the April-September period, citing plummeting truck sales attributed to the serious economic slump in the domestic market.The Nissan-affiliated company reported a midterm pretax loss of 6.2 billion yen and a net loss of 9 billion yen, projecting a 13 billion yen pretax loss for the whole of fiscal 1998.For the first time in five years, the company now projects that it will suffer a net loss of 3 billion yen for fiscal 1998, Nissan Diesel officials said. In the first half, the nation's truck sales fell 36.4 percent to 39,269 units, compared with the same period of the previous year.