The rubbers on mine are also hard and cracked, but that is not where the leak on mine came from. It was actually from the rubber edging on the body. Now let me try to explain what I mean. You've got the rubber on the lid, right. Then you've got the edge of the trunk itself that is pointing up. The rubber of the lid comes down around this edge on the body. This edge has a rubber that looks just like a length of thin hose cut open along it's length, which is fitted to the forward edge. The side edges has something similar, but made of a steel re-enforced (?) plastic edging which is much more durable. In my case the forward edging came loose at a corner, and when closing the lid it would sit diagonal across the corner. I've replaced it with proper edging, which is also taller. Without it, the gap is too large and water blows over the edge into the trunk.

There are also hoses (1 or 2) on the forward corners inside the trunk, coming down from the rear window glass. Can't now remember exactly what it's for, but it might also be leaking?

Have fun...