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had same prob in my s2 was leak around rear lights check all terminals for corrosion as it causes your lights to do all sorts of weird things when you brake etc
Had a similar problem with my Sapph, get some sealant from halfords. Best get automotive grade stuff as it withstands oil, grease, fuel, higher temperatures, etc... than your average bathroom silicone. Also it comes in black which is handy.
take the back lights out replace with new gaskits and a very little smear of gaskit sealant will do you a good un,i had this on my s2,took me ages to find it as well