1996 Mercury Mystique coolant system need desperate help to figure this one out?

On an old car I used to have, the exact same thing happened and it was the Heater Core, which in the vehicle I had was right behind the firewall. As a quick fix, you could try a bottle of "stop leak" that MAY plug any minor cracks/holes, if you keep smelling it after that, you may need to change out the heater core?.