Another door lock question

[D_Ryan]

Good Evening All;

A search of past posts fails to turn up one with the same issue, so I need to ask for a consensus on the cause of my non-operational passenger side front door lock... Vehicle is a 1985 300TD.

This is not a lock cylinder which will not turn; rather the key inserts just fine and the cylinder turns without issue. The problem is that turning the key/cylinder does NOT release the door locking mechanism. The only way to lock/unlock the door is via the locking knob on the inside of the door.

The cylinder did work without incident, then suddenly no longer did, as if the connecting rod is no longer engaged.

I pulled the door panel, but it is difficult to see up into that area of the door to get an understanding of how the rotation of the lock cylinder manipulates the door locking mechanism.

What is the likely cause of failure, and the best fix?

Thanks,
Dave