With regards to changing yout light I'm assuming you mean one of the rear light clusters on the sides and not the centre brake light? In which case, remove the two screws, then inside the boot fold back the small section of carpet at the rear of the light for access to a plastic wing nut - this retains the light unit in position. It's not something you'd find unless you were looking for it and rather helpfully Ford didn't think to mention it in the handbook!