Heater core leak, right? Before replacing the heater core (major job, the dash board has to come out) check your heater control valve. It is located right under the air cleaner housing. When it leaks it sometimes drips down onto the exhaust system—which is why you may not find spots on the floor—and because it sits inside the heat shields surrounding the cats, it burns off when you start the car.

We have even had one that fogged the windows. That sure sounds like a heater core, but upon further investigation we found that when it was burning off, the cats the smoke would make its way up the drains to the area right behind the firewall, at the base of the windshield (sometimes there is a high intensity washer bottle there). This is where the heater takes in fresh air, right at the base of the windshield, with a plastic rain shield. When it rains the water has to go somewhere, usually down the drains. The steam makes its way up and if the heat is on, it will pull it right inside the cabin.
I wouldn’t have believed it if I hadn’t see it for myself.