Sales of minivehicles in Japan fell 16.3 percent in September from a year earlier to 123,945 units, marking the 12th straight month of decline, the Japan Mini Vehicles Association said Tuesday.Minivehicle sales during the first half of fiscal 1998 through September totaled 686,672, down 7.7 percent from the same period last year and marking the third straight year that sales have shown a year-on-year decline in the first half, the association said. Minivehicles are defined as any passenger car, van or truck with an engine capacity of between 550cc and 660cc.In September, sales of minicars totaled 67,373, down 18.7 percent, while those of minitrucks and minivans fell 13.3 percent to 56,572. In the first half, minicar sales totaled 388,862, down 7.5 percent, and minitruck and minivan sales were down 7.9 percent to 297,810.