The aluminum tube needs to be flared at each end in order for it to seat correctly in the transmission housing.
The first photo shows how the tube flare sits into a groove in the drive cable housing and how the rotary motion of the motor is converted to the back-and-forth motion of the drive cable, just like a piston in a cylinder. Different holes in the disc will give different sweep angles of the wiper blades.
The second photo is a closeup of the wiper transmission with the backing plate removed. The tubing flare sits into a groove in the transmission halves and is locked into position when the halves are bolted together. Also seen is the drive cable meshing with the wiper drive gear. This whole assembly gets greased liberally so that the cable slides back and forth easily inside the tubing.
