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It's usually obvious which is the earth pin once you've removed the bulb holder from the rear of the lamp. On most cars, you can just trace the ground rail back from one of the bulb holders by eye, but if all else fails, a multimeter (with all bulbs removed) will give the game away.
If there is an earth fault and it's not on the connector block, then the simplest solution would be to wire up another earth in parallel. The screws that hold the boot lid striker plate to the car body are usually pretty effective earthing points in my experience.
Edit: A further thought - if the design of the Megane is anything like the Laguna I, the earthing for the lights is bolted to the car body very close to the light cluster. It could be a simple case of the screw working loose.
Currently in Horatio's garage:
2010 Laguna III Initiale.