FIXED 100%
I had the same problem on my Signum (same as a Vectra) - rear footwell filling with water and front footwell damp.
It had me foxed. It made no sense and there was no obvious sign where the water was entering (roof?, door? windows?). It was none of these.
Problem was fixed as follows:
Open bonnet. Stand at the passenger side. Identify the black plastic cover that runs along below the windscreen (has washer jets in it). The centre section (almost the full width) is separate from the two end pieces.
Ease the front edge of the center section (furthest from the windscreen) out of the rubber seal. No need to go alll along, just as far as the centre.
If you look down you will see the heat exchanger (a vertical silver radiator that is to the passenger side of centre).
Look below this at the floor of the cavity and you will see it slopes down to a point below the centre of the heat exchanger. There is a black rubber "funnel" set into this. It is the drain for any water that collects in the heat exchanger cavity (condensation or rain). It shouild be clear of any debris but it will be clogged up with mud and decaying leaves, etc. Clear it out.
I used a flexible rod that I have got (it is for unblocking sinks) to carefully unbung it without damage and without having to reach inside. Anything will do, but take care not to damage the rubber funnel.
I then flushed it clear by pouring a jug of hot soapy water into the thing until there was no dirty crud. (it all pours out under the car).
Now push the cover back, making sure that it fits back .snugly under the edge of the rubber weather strip.
No more leaks into the footwell.
To remove the water in the car, the best thing is a wet and dry vaccuum cleaner. If you have a clear upholstery head you can see the water being sucked up. There is a lot of sound insulation material under the carpet, so the process will take some time. Don't waste time trying to dry it out with fan heater or similar, it won't do the job, will make the air damp and you might damage upholtery and other bits.
The relative humidity in the car will be high, so wipe windows and windscreen and other cold surfaces to remove any condensed water.
When you drive put the aircon on and switch it to recirculate and it will help to remove the moisture in the air.
The result is a car with no leaks, dry carpets and no damp smell.
I hope that this helps.
How does the water get in? Well, when the rubber funnel is blocked and it rains, the water is prevented from flowing away and fills up in the cavity and then flows into the openings for cables. It then runs into the car (under the carpets) and finds its way to the lowest point which is the rear footwell. I know where it starts and where it ends up, but have not bothered to find the route.
It does not take long to do. Apart from a stick or flexible rod to clean out the gunge and a jug of soapy water, you don't need any tools, and anyone can do it. Unbunging took about 5 minutes max. Vaccuuming the carpet took about 1 hour until it was dry - that was the tedious bit and a lot of water came out.
We had very heavy rain the next day and the carpet was bone dry.
Many thanks to Geoff, Steve and the others who suggested the solution.
Bob, December 2012