1999 Toyota Corolla Coolant smell in cabin; Heater doesn't

I understand that this sounds exactly like a heater core thing, but here is the history.
This car has been in the family since new, and the heater has never really worked. Lately, I have been smelling coolant while driving to work with the heater on. At first it was only while the knob was in "WARM", but now it happens regardless. Any clues?