I've been going over this scrote-ache for the last few months - it turns out that my '07 400h that I got last year was cheap because it doubles as a water carrier! So far I have replaced the boot seal/rubber as it was a bit mullered, sealed the upper panel seams leading down from the rear roof-rail fixings & as of yesterday put some sealant around the holes where the plastic trim fixes next to the rear light clusers. I had leaks on both sides originally, but after 3 hours, a socket set, screwdriver & 2 litres of water in a bottle I'm sure that I've nailed the passenger side and will keep an eye on the driver's side. I want it to rain now so I can go and see if I finally can take towels out of the back of my luxury car and put my damn boot back together! 😄