UPDATE: For months I've been manually locking and unlocking my front passenger door (started working randomly) and had the actuator disconnected. Today I got some unrelated replacement parts for that door, and planned on reconnecting the actuator since it seemed to be working, but after connecting the actuator I couldn't lock again. Figuring the lock/latch was bad, I pulled it only to find that its actually a fairly simple assembly that appears to be functioning flawlessly.

After I disconnected the linkage from what seems to connect to the door lock and indirectly the actuator, I had no problem locking the door manually. NOW heres whats weird, that door lock has always been different from the others. The travel of the locking pin (the one that protrudes from the door) has always been less than all the other doors in the car. When unlocked, the pin was always significantly lower than the other doors. When locked, the pin was slightly lower than the others but not as much. Because of this, I figured the lock was defective, but upon inspection there is clearly nothing broken there. I can't think of any variable that would lead to the different in pin travel as the lock mounting position does not vary inside the door (and I have now mounted it WITHOUT connecting the internal linkage which I guess is related to the emergency unlock) On realoem, I noticed that the locking mechanism part only ran to 11/88, does anyone know what changed after 11/88. I'm suspecting that this might simply be a different lock from the others hence the linkages and all being different and causing problems. Else, the problem would have to be the solenoid-like thing connected to the lock cylinder for emergency unlock, but that would still not explain the pin travel.

Sorry that this post is so lengthy but this is difficult to describe, i'll try to get pictures to illustrate what I'm saying.