Do you mean that your heater core is leaking coolant ? That would mean that the heater core is clogged. The heater core is like a mini-rad, if it is clogged it leaks.
On a 1996 Mercury Sable : The thread on cap on the engine coolant reservoir is the pressure cap / radiator cap
The heater core could be clogged up.
Coolant is added to a 1996 Hyundai Sonata by opening the coolant overflow tank and adding coolant until the full mark is reached. This allows the cooling system to pull in coolant as needed from the reserve tank.
Heater core.
The coolant level sensor is bad. It is integrated into the coolant bottle, so you need to replace the coolant bottle.
chances are that you have a blockage somewhere heater core radiator heater hose try flushing the system first then go from there
check/replace the engine coolant level sensor located in the engine coolant overflow reservior (located at the rear of the engine compartment pass. side)
I'm having to guess it's the low coolant light? Possible coolant switch in the overflow bottle is dirty or defective., Is it low on coolant? Possible wiring issue to sensor.
LOw coolant? Weak thermostat?
Heater Core
Is it not hot? Not blowing? Leaking coolant? Need more info to help you.
Low coolant? Thermostat not opening?