i had the same problem with my 95 omega. the problem is your drain plug is probly blocked, to fix this open your bonnet and remove the plastic trim under your windscreen, you will see all of the wires that go into the cabin the rubber gromit is more than likley disengaged. at this point you will probably see a puddle of water just sitting in there, get a long screw driver and give it a wiggle around at the bottom of the puddle this should clear it ,if not try and prise the drain plug out ( i did this) the water should run out and you wont get this problem again. i left the plug out pointless having it block again. to dry your footwell out pull up the carpet and put loads of cat litter under it leave it a day then hoover it out.
adz kennedy
May 2008