I have an October 2006 Vectra and recently found the passenger side carpets were sodden.
The problem i9s as described for the Omega - the drain tubes near the wiopers under the bonnet had become blocked and the "sumps" were full of water. By simply sticking my finger into the drain tubes (after lifting the grill up) the blocakge was cleared and a litre or so of water was released.
Carpets dried put using the hose attachment on a domestic carpet cleaner and then a fan heater (plus some spray disinfectant)
It appears Vauxhalls of all models have this problem and of a large age range
Ian Robins
February 2010