If you're smelling coolant through the heater and you don't find any leaks around the heater core, then your next bet would be that you may have an intake gasket leaking. When your intake gasket leaks, it burns off of the exhaust and that steam gets sucked into your heater from the blower. I would stop driving the car if you have an intake leak and get that fixed asap.
My car is at that shop now having my lower intake gasket replaced and how i knew it was going bad? I smelled it coming in through the heater. So i took it to Pep Boys for cooling system diagnostic where they stick die in your cooling system and sure enough, lower intake manifold gasket leak.
Car 93k mi., engine 83k.