Vauxhall Signum massive water leak

Try the window seals. the rear lights usually have a thin rubber around them to stop water so check them for damage

Hi had the exact problem on my wife's car and took me ages to find,its a nissan micra,nissan could not find it,I could not understand how my boot and rear foot well could get so wet at the same time,I got into the boot and got the wife to put a hose on it,the water flooded in down the inside,it flowed down the back of the rear seat into the foot well and into the well in the back onto the spare tire,lifted the hatch up and looked at the top of it,there was a large rubber grommet and sleeve with wires inside going into the hatch door and the car body,the water was getting in through the grommet on the car body and dropping down in between the metal skins,I pulled the grommet out of the car body and applied clear silicon seal pushed it back in and put some around the grommet it self,left it over night to go off,now completely dry,and no more steaming up ,which was a nightmare,could have the same fault,good luck tony

just to say my wife's car is a 2010 and nissan would not fix the grommet as not on the warranty,because its a rubber seal which is not covered because its a perishable,I was annoyed at that and would not let her pay for the repair ,they also said it would take two days ,I was annoyed at that too tony

Terrible service from Nissan . A grommet doesn't deteriorate in that length of time ( takes many years , if at all ) Plus it was due to not sealing correctly not a faulty grommet .

Have you had a look at this similar problem:

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Under the bonnet to the rear right of the engine compartment (under the passenger windscreen area) there is a plastic sleeve that is a drain for the window gutters, however this gets blocked or frozen and the water subsequently runs into vehicle. Just locate it (looks like a black sock) and clear out with fingers. Job done !!

I have noticed that on the inside of the boot next to the left side boot ram there is a protective cover for cables going in to the boot space. you will probably find that the seal is not seated to the car correctly. You will have to use a silicone to seal both sides of the cable protector. If you put your hand up the back of the light on the left you will notice that the soundproof material is soaked witch is not the case on the right hand side. It will take a while for this to dry out completely. Note that if there is no dampness behind the light then I dont know if this fix will work but it is a common fault!! Please reply if this had fixed the problem. It had worked on a number of signums and vectras.