Cadillac Repair: 91 Cadillac lock issues, cadillac deville, couple questions

Question
What needs to be replaced when the door doesn't lock on my 91 cadillac deville? I can press the lcok button but the door opens right up. How can i fix this. What do i need?

Answer
Martinique,
First, a couple questions:
1. What door are we talking about? I assume the driver's door but want to be sure.
2. Do the power locks work? In other words, if you press the switch, do all the locks appear to move into the locked position.
3. If you manually lock the door in question, either by using the key on the outside of the door, or using the lock lever on the inside of the door, does the door actually lock?