I had a full fish tank in both my rear light clusters on my facelift. It really used to pi$$ me off.
Solution, remove plastic covers from inside hatch. unplug wiring connectors, undue the nuts, leave one in place on each light, Now the difficult bit there is a screw holding the lights in from underneath. the only way to get at it is either remove the rear bumper or a very long extension from underneath which I did. Once you have removed the screws take the last nuts of and remove lights. shake out the water and drill a few small holes to allow the water to remove, remember to have a sharp drill and drill very slowly or you will have an unsightly burr on the inside which is visable. Leave the lights near a radiator to dry out and refit.