You can always shut yourself inside the boot, have a friend outside (key ready off course) run water from a hose pipe around the boot lid and rear screen (upper) region and see where the water is getting in from, also pour buckets of water as sometimes the leak needs pressure (i.e. travelling on rainy day on motorway and water is pushed into the seams with force) I had a simular thing going on many many years back on a peugeot 406, and found it was the seal. just removed it, and put it back on, solved the problem, seems to have sunk too low, well worn in fact. it was an old car so never thought of replaceing any thing on it.