1997 Chevy Monte Carlo Heater Core

Hi- My car is leaking coolant from an orifice that is up against the firewall on the passenger side. Can you tell me what it is leaking from. My Heater works & blows hot air. My A/C works and blows cold air. Recently replaced Thermostat, and then used a Radiator cleaning solution made by Prestone before I flushed Radiator. However, I didn't flush entire cooling system.