it might be worth checking that you haven't got a leak somewhere - door/window seals roof is closing properly and that the drain holes are clear, there are several little drains around the sealing lip that easily get blocked with crud.. you can rinse them out with a hose and something to prod it i have used kebab (long cocktail) sticks... there are also several on the scuttle - the area where the wipers come out behind the bonnet. feel around on the carpets, floor side panels for wet areas - evidence that water might be coming in and go up from there if you find anything... i think if it was coolant you would smell it and the windows would steam up when you have the heater on...
Oh also check the air-con drains are clear, remember there are two one in the front and one in the rear.. the car should leave two wet patches on the dry road when left running with air-con on - or after a trip and parked up for a few minutes.