The most common cause is if the windscreen has been changed on the car recently or in the past, either the appature has rusted around the screen due to a poor installation or the urathane sealant has not made a proper seal.
If the screen was changed recently then contact the installer.
If there are signs of rust around an older screen it will need a body shop.
The other main cause can be a damaged or rotten door seal. A good scrap yard should have one but check it carefully first, or try a Vauxhall main dealer but this will be expensive.
One other possibility is the gater that seals the bulkhead where the main loom comes into the car through the bulkhead. If this is worn or perrished it can have the same effect.
Good luck. Sorry for poor spelling!
boogie, January 2006
