Wiper motors are usually mounted on a metal plate that is mounted to the body on rubber mounts to stop the transmission of vibration. Since the rubber insulates it electrically too, a ground strap is needed to complete the circuit. It is typically riveted to the metal plate and run under one of the bolt heads. Under the bolt head is where it arcs and loses its connection. You can prove that by connecting the ground clip of a test light right to the battery negative post, then touching the probe to the case of the motor while the wiper switch is turned to "low" or "high". If the test light lights up, the motor isn't grounded. The fix is to attach a new wire between the motor or mounting plate and to the body.