As above--check the screws which hold the rear light clusters in place first of all. The other common leak point is the recess in the bodywork where the tailgate hinges bolt into, at the top of the hatch opening, one either side. There's an assembly fault which leaves a hairline gap between the seams, not really visible but water gets in there and tracks down into the boot. Smear generously with silicone sealant all around this recess to fix it, after cleaning out the gunge. This fault caused my boot to fill with water and took me ages to find.

Thank you