I just bought a 1967 nuova500, and need to repair or replace the door lock on the driver's side ASAP, as it is parked in a public place. The button on the inside will not push down fully, and the key turns on the outside, but does not lock the door. The passenger door seems to work OK. From looking at it, it seems I must remove the panels covering the inside of the doors, and see what I can figure out from there. Any suggestions or your previous experiences appreciated.

Remove the interior door handle and window winder handle.
Then take the door card off.
Lock is easily removed with three screws.....would probably benefit from a degrease and clean.

I agree with Andrew---remove the door card (be careful, there is a retaining clip in the middle of the front of the card and the middle of the rear--they are easily broken!) and clean and check the lock mechanism. If the locking button won't go down, and the key won't lock it I would say that there is a fair chance that the linkage is fouling somewhere.