Well, you can say this about the Merrimack River: It has been, historically, a practically perfect place to dump a car. Water the color of black coffee. Dozens of secluded access points, especially along a wooded trail that runs beside the banks from Lowell to Lawrence.In this state, with its grand tradition of boosting cars and committing insurance fraud, you’d be hard pressed to find a better spot to vanish a ride. So far, crews have found nearly 80 on the river bottom, cars that long ago passed from transportation to evidence.