Here is more info on how that cam assembly works based on what I noodled....
The MAIN part in the cam assembly is the SPRING. It is responsible for grabbing at first to rotate the cam around to lengthen or shorten the arm so it can park /wipe as needed.
When the motor gets turned on, the cam assembly starts rotating with the crank...initially (from a park position) it travels a short distance with the spring riding on the shaft tightly which rotates the cam plastic pieces. When it all reaches the end of the designed travel range, the linkage length has been changed to allow wiping, and the spring then opens a bit to allow the shaft to rotate in forward mode freely.......this stays this way while they are on.
When you turn off the wipers, the motor immediately rotates in the opposite direction..... the crank shaft obviously reverses direction as well. It again travels a short distance with the spring riding on it tightly which rotates the cam plastic pieces again, changing the length of the wiper linkage, allowing the wipers to come into park position.
The spring on mine was worn and the shaft was as well.....that means that when the little inner block on the cam plastic parts would push against the spring, it would just rotate, which means the cam would not rotate which in turn means the eccentric cam didnt move which then means the linkage didnt change length to park.
With the new spring installed, it grabs the shaft of the crank lever tightly at first so the cam rotates.....then as the innards reach their end of range, the spring loosens up so the shaft can rotate freely.
Its a neat design if not the most mechanically long lasting...I can see how over time the spring and shaft will wear , leading to slippage and then a non-park situation.
As noted above, I had wrapped one layer of electrical tape around the shaft and reassembled and things started working better.....so I knew I was on the right path.
If someone could figure out a way to slide some sort of bushing over the shaft to effectively tighten the spring and shaft fit that might work.