Lock cylinder slides into cylinder case (sleeve). Rear of lock cylinder protrudes into door lock rod's right "odd" shaped hole (inside of door), and is retained to the lock rod with a metal C-clip.
Another lock rod comes from the door latch assembly to the door lock rod, it is bent at a 90 degree angle and the end of this rod is inserted into the left "round" hole of the lock rod. This rod, is retained by the lock rod retainer, which is plastic, but it does nothing other than to retain the rod to the latch to the door lock rod. If this piece of plastic broke, the only thing that would happen is the latch rod would slip out of the door lock rod.
The metal C-clip is what retains the cylinder to the door lock rod, and is the same type of metal C-clip used on every vehicle (GM or otherwise), I have ever worked on.
Not shown on the drawing is the plastic retainer that is inserted at the lock manufacturer plant that holds the lock cylinder to the cylinder case for shipment to the assembly plant. It is not shown, since it is not required as a serviceable part when a technician performs any repairs on the lock cylinder.
All that plastic retainer is used for is to keep the parts together for ease of assembly, so at the plant, the entire assembly (lock cylinder and cylinder case) are inserted into the door, through the lock rod, and then the metal C-clip is attached, along with the other rods.
So, if I used a slide hammer, I would pull the entire assembly out, and when it fell on the ground, the plastic retainer would be there. For a untrained person, this might make it appear the cylinder was held in by the plastic clip, and it was, for assembly.
