It is a common problem for leaks around the lower edge of the rear windscreen. The usual reason is that the rear windscreen trim fittings work loose and water finds a way round them through their fitting holes into the boot.
I could feel the loose fittings by prodding my finger through holes in the frame in the boot below the windscreen. I could feel that the problem fittings were wet with drips of water.
These leaks if undetected ultimately fill the spare wheel well with water causing havoc with any electrical equipment located there.
After making some enquiries on the usual forums asking for some advice as to a cure it was suggested I tried to “soak” the fittings with a liberal dose of Capt. Tolley that worked for him. The alternative is to remove the windscreen trim and replace the fittings with new ….. risking bending the trim. Thought I’d get the Capt.Tolley method first.
Well after several heavy rain storms and several washes so far so good ……. fingers crossed. It was also suggested to repeat the procedure annually.
More about water leaks …… https://mgzt.info/category/bodywork/water-leaks/