Talk to Maguire (the boy leaning against the wall) about Peagram and the dig until he tells you Sean was working there. Enter the bar, snag the towel off the bar then talk to the man with the cold in the far right corner about his wire. Afterwards, pick up the wire while he's sneezing. Go talk to the drunk under whose arm you found the towel about Sean. Order a beer for him to get him to open up. Now that you have Sean's work history confirmed, go and talk to him. He's the fellow sitting alone at a table in the middle of the room. Quiz him about Peagram and his package until he runs out and gets hit by a car.
Go outside and fiddle with the box whose lid has come off in the accident. When you've broken the switch, go back into the bar and talk to the bartender. Request a beer from him, then show him your electrician's ID card so he'll let you behind the bar. Use the wire you got from the trapper on the outlet above the dishwasher. When the bartender lets you down into the cellar to fix the pumps, just flip the lever on the wall and go back upstairs and out the front door. Now you can open the grate in the sidewalk. Head back downstairs and you'll get Peagram's gem, and a flashlight. Eventually you'll need to soak the towel in the sink, but in the PC version of the game the towel will dry out, so only get it wet if you're playing on the Game Boy.
Outside the bar again, head down the path in the top right corner of the area. Talk to the man at the haystack until he leaves to celebrate his nephew's kidnapping. Climb the haystack and use the manhole cover tool in the wall to create an impromptu step for yourself. Once you get into the courtyard beyond, you will see an ornery goat. In the Game Boy version you can go pull the plow over right away. If you're on the PC, then make an attempt at the ladder descending into the ground to entice the goat to attack. Before the goat gets back into position, quickly click the plow so George will run over to it. Either way, the goat should now have his rope tangled up in the plow, allowing you access to the ladder.
Go down the hole and get the Plaster of Paris from the bag sitting on the table in the far right corner. The plaster is used to create a mold for use with the mechanism to the left of the door. Below the step up to the door is a small statue you will need to knock over, then pick back up. This will leave five impressions in the soft sand which you can fill with the plaster. You need to mix the plaster with water, so squeeze the towel out over the mold. If you're on the PC version, you'll need to run as fast as you can from the basement sink back to the plaster. Click out of any dialogue on your way.
Once you have the hardened mold, pick it up and stick it in the wall to the left of the door. The door will open and you can now descend the steps. There is a cutscene here, after which you will find yourself back in Nicole's apartment in France.