I had door problems galore on my previous 1989 SSE. The problem is- once theose door latched sieze, the whole mechanism must be repalced. Happy to hear that you got into the car though. On your other doors- remove the door panels and plastic film, and now you should be looking at the inside guts of the door. put your hand inside the door and feel around for a triangular nub sticking out of the latch. spray the **** out of this piece with oil if it won't move. other tip, try locking and un-locking the door a bunch of times, this also frees up the latch. moving the triangular piece up will open the latch.
Once the door is open- the rest is straight forward. remove the three torks bolts in the frame of the door, un-do the metal arms attahed to the latch, and remove the old one. I suggest covering the new latch in wheel bearing greese. This will displace all the moisture and prevent future sieze-ups.