I have antifreeze leaking by the back fire wall. I think it could possibly be the heater core, but I have found no antifreeze in the car.
The most common spot for coolant to leak near the firewall is indeed the heater core. The coolant is coming from the evaporator drain hose. The cooling system should be pressurized to verify this leak. Most of the time the coolant will not leak into this car because of the design of evaporator case. The replacement procedure will require some dash removal, so you may want to enlist the help of a certified mechanic, like one from YourMechanic.
Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing