heater core leaking
Its more than likely your heater core.
If there is coolant on an engine, it is likely a seal or a hose is leaking or broken. It should be repaired immediately or the engine may seize up and quick working.
Bad hose or clamp where the heater core is connected would be the most likely problem.
Freeze plugs. More likely that hoses are leaking or thermostat housing is leaking. Freeze Plugs generally NEVER leaks unless engine block has had frozen coolant in it. Hence, Freeze Plugs.
If coolant is leaking from the back of the engine it is most likely that the intake gaskets need replacement or a possible heater hose connection depending on the year.
If it is an automatic transmission, the cooler in the radiator is most likely leaking into the coolant system.
You need to explain more on where exactly the coolant is leaking. There are several possible reasons why your coolant is leaking. If it's from your main hoses that lead to and from the engine or radiator, you need to check the condition the hoses are in. If they're in a decent condition, it's not that. If it's leaking around where the hoses are connecting, it's probably not tight enough, or there is possibly a clog in your coolant line. Also get your radiator checked. If it's leaking into the passenger compartment, it's most likely the heater core.
most likely heater core leaking
Leaking or loosing coolant? If it is leaking then where is the leak? If it is just loosing coolant with no apparent leak, then you more than likely have a blown head gasket or cracked head. If there is coolant in the oil, or white sweet smelling smoke coming from the exhaust you definitely have an internal problem. Serious engine damage will occur if you continue to drive this vehicle.
If you aren't seeing oil leaking in the driveway then it almost has to be burning it due to worn valves or pistons. There most likely is exhaust smoke but you just can't see it yet. One other thing to check is your engine coolant. A leaking main seal will allow engine oil into the coolant side of the engine which will turn your coolant a milky white frothy looking liquid.
Find the source of the leak. Most likely areas are a brake line at the wheels, or the master cylinder at the upper part of the firewall in the engine compartment.
Most likely it's a ruptured heater core, leaking coolant. But before you replace the heater core, check the connections outside the firewall. Sometimes a leaky connection will drip to the inside... but don't expect it; it doesn't happen often.