We have a 2005 SE version that had water in the boot when we bought it we innocently thought that the boot had been open for some reason at the time as it was raining heavy for a while , yes we can be gullible . But as we had had it a few days we still had this problem even after i had mopped it all up previous , it was getting worrying, i had the boot seal off, the back lights off , and the roof trim removed and cleaned the guttering there but couldn't trace it, it looked all fine then a few day's back on a dry day i got the hosepipe out and worked a slow stream of water from the lights up nothing ! Arggh, then i focused near the rear of the roof trim in the gutter where the rubberised strip lies right at the rear near the boot hinge the water appeared " got you " i thought YES there was a hairline crack in the mastic seal next to the rubber boot supplying the electrics to the boot hatch and same on the opposite side .
I applied some caravan mastic and it solved the water ingress so from my experience check this area first as it was not obvious and hard to get to.