Help me figure out with door lock mechanism is faulty please

[Jinster]

I know I have a problem with one of my door lock mechanisms. But I can't figure out which one.

I have a MY97. No remote. No alarm.



When I try to lock from the driver side with 1 turn of the key:
the driver side locks and unlocks
the passenger side does nothing (stays unlocked)


When I try to lock from the driver side with 3 turns:
the driver side locks
the passenger side does nothing (stays unlocked)




When I try to lock from the passenger side with 1 turn:
the passenger side locks
the driver side locks and then unlocks


When I try to lock from the passenger side with 3 turns:
the passenger side locks
the driver side locks



So which side is the fault? (when locking from driver's side, nothing happening to passenger side seems to suggest passenger side problem; but able to lock up the passenger side from the passenger side with the driver's side locking and unlocking seem to suggest driver side problem)




P.S: Both windows courtesy drop when doors are open, so the door-open sensing part is at least ok.