I only have experience with the Freightliner FS65 Thomad build. However, on t=our bus, the wiper arms attach to the wiper motor by sliding down a spline on the wiper motor shaft and a similar spline in the arm. Thus, there are a number of positions the blades can be set.
When I replaced my wiper arms, I did not get them on in the same position as they were originally set, so the driver blade did not rest the same place. To fix that, I loosened the nut, moved the arm away from the motor shaft until it cleared the splines, rotated the blade in the direction needed, and slipped it back down the wiper motor shaft.