Volvo V70/XC70 Rear Doors Locked

I purchased my car (2002 Volvo XC70 D5) in December from a garage. The off-side rear door would not open - from inside or outside - even though the locking button went up and down. Their service dept. fixed the problem before I took delivery. However, it jammed again after a few days, quickly followed by the near-side rear door. The garage replaced the door lock assembly in the off-side door, and lubricated the lock in the near-side. A few weeks later, both sides failed again.
My local garage removed the near-side door card, and opened the door. The lock assembly was replaced with a good second-hand one. It worked fine for a few days - with the door card left off. Then it went back for the mechanic to look at the off-side door. He replaced the door card on the near side, and immediately after relocking then unlocking the door, it was jammed shut again. The door card of the off-side door was removed, the door opens and shuts, locks and unlocks fine, so we have left the card off.
In summary, it seems that the door locks will not operate properly with the door cards in place. Does anyone have any experience of this, and any ideas how to fix it please?