I had a long term leaking issue on my 2010 Omega. During any decent rain it would leak into the front passenger footwell which would become saturates and when car was parked uphill, it would flow under the carpet and saturate the rear passenger side footwell.
I pulled the passenger side component of the cowel off and found a missing gromet in the wall of the drain coming off the windscreen. The whole lead straight into the passenger footwell so I placed a gromet (and some Sikaflex for good measure) and thought the problem would be solved.
Next time it rained heavily the passenger footwell became saturated again.. I pulled the wipers and the entire cowel off to get a good look. I found that where the aircon intake plastics were attached to the firewall, it was loose and the seal at the top had a 1-2mm gap along its whole length. The water would leak behind the plastics and through the aircons while in the firewall. I smothered the top and as far around the sides as I could with Sikaflex and the problem is solved!
In the photo below you can see the gromet I replaced in the bottom right, and the line of white Sikaflex above the aircon air intake.