Another passenger door lock issue
I'm having passenger door lock issues with my 1985 300D. Sounds like it might be a similar issue.
The driver's door has the vacuum master switch...you have posted a picture of this device (black cylinder with 3 rubber hoses attached). Both rear doors have a mechanism that only allows lock/unlock based on the vacuum circuit.
The passenger front door has a vacuum control, but it also has a spring mechanism that allows for a manual override of the vacuum circuit (with the key or the inside plunger). Mine has never worked; when I took the door apart I found that some of the spring mechanism is broken/missing. Go look at my thread to see a picture of the top of the rod from the vacuum device. Your picture of the passenger door doesn't quite show the linkage; its where the top of this rod connects to a 90 degree bellcrank