Truck maker Navistar International Corp. said today that its net income for the fiscal third quarter ended July 31 was $18 million (or 26 cents per share), compared with a loss of $16 million a year earlier. Sales were $1.89 billion in the quarter compared with $1.59 billion, the company said in a release.Daniel Ustian, Navistar chief executive officer, said that the profit was achieved despite continued softness in medium-duty truck shipments. However, he said he was encouraged by strong pickup in new medium truck orders in July.Preliminary July Class 6-7 medium truck orders totaled 7,700 units, up 37% over a year ago and the highest monthly total since August 2002, Navistar said.