i think this may help a member posted this
Water leaking into footwells - new solution
After a few months of going crazy with my vectra c leaking rain water into both footwells after every downpour i finally found the problem:
When my car had been standing in rain,water seem to be pooling somewhere behind the dash then leaking into both footwells when driving off. I could hear water sloshing from side to side as i drove. Took both "L" shaped panels off in both footwells to expose ecu/wiring and water could be seen pouring down both sides behind soundproofing, eventually seeping under carpets and soaking them through. Before I removed these panels you can't notice the leak as it cunningly leaks hidden to below carpets.
Most people told me it was common for the vectra to leak through the pollen filter into heaters and through pipes into footwells caused by blocked drains in scuttle panel - this wasn't an issue in mine as it drained how it should do!
Next i decided to have a windsreen "accident" thinking a seal may have gone, got the screen replaced but made no difference!
Then i noticed this gap after removing wipers and scuttle panel top cover. Just below where this panel clips to windscreen there is a gap where windscreen ends and bodywork starts - only a few mm wide but runs width of screen and opens out slightly towards edges. Anyway seems water runs down edges of windscreen and can enter this void, then sit patiently there until u drive off and somehow enter the footwells.
All i did was fill this gap in with silicon sealant from edge to edge to make it water tight and TA-DAH not leaked since!!!!