Water leak through back light ???

Hello my wife's car is letting in water to the boot and also in to the back passenger footwell,its a two door,the water is running from the back right side along the edge and over the edge to under the back seat,tried water hose on the car definitely going behind the light cluster and into the boot,there is nothing behind the light cluster to stop water fro getting in ,just a thin piece of foam rubber no matter how tight you screw it down it still gets past,if I see the cluster I wont be able to get it off to replace a bulb,has any one got any ideas please,is this a common fault,nissan say no,they say they wont the car for three days to have a look ,too see if it is the light cluster,(it is )why should I have to pay over £100 for them to agree and then fix,but how can they fix it,should there be a rubber seal,is there something missing,the car has 12 months nissan warranty left ,but they are saying that does not include any seals at all on the car,all seal are classed as perishable so not covered ,thanks