How to change a Ford Focus Reverse light bulb?

My 2002 Focus Hatchback reverse light has stopped working, and the light compartment, located in the rear bumper at the rear, is full of water.

Perhaps I'm stupid, but I can't work out from looking in the manual how to get into the compartment housing the reversing light.

How can I remove the bulb compartment to drain the water, reseal, and replace the bulb? There seems to be no obvious catches to release the bulb compartment, I've looked behind it (ie behind the rear left wheel) but could find nothing! The exterior of the bumper also has no obvious catches around the edges of the light housing.

If anyone can post 'an idiot's guide', I'd be a very grateful idiot!

Thanks in advance