Iv been suffering with this problem for ages (iv posted a few links about it...) mine has got to the point where the rear sound proofing has rotten due to it being perminantly wet...i have however poss fixed it...after spending 475 quid at a bmw garage to try and locate it...which they failed to do!!...i lost my temper and siliconed the entire rubber seal around the rear hatch (i only get the water in the rear cubby holes and under the tool kit by the spare wheel)..as i found the water appeared to be coming in from a weld behind the rear lights....
Remove the rear lights..make sure the entire area is dry...keep the rear hatch open and pour water down the channel by the boot struts..in theory it should just pour off..mine howeva was leaking into the boot...therefore gathering in the cubby holes and being soaked up by the soundproofing material...which in tern rotted which made my car smell like a dead animal all the time...hummmmmm yummy!!
Since siliconing the around the entire seal it appears to have stayed dry..whooooo hooo!! I still cant work out how the water got in and under the seal as the rubber sits very tight on a thin metal lip///but frankly i dont care it appears to have worked so im a happy chappy..the leak has been the bain of my life..especially with our cruddy English weather!!!
