A common problem and one I suspected was leaking on mine is the original type gasket used on the rear light cluster. The original gasket had a split as can be seen in the photo which can leak – replacements are one piece with no split so no potential leak spot.
The one piece gaskets are readily available and easy to fit. It means getting access to the back of the cluster to get to the fixing screws – the same as if you were replacing a bulb.
More possible water leaks – see … water leaks – MG ZT cars info news and reviews