Inspection

• First, check if the antifreeze cannot go through the heater core as it should. If it cannot pass through, it will simply leak out. Antifreeze leakage will usually collect in the floorboard, passenger side. You can also check for this by looking for a sweet scent while the heater is running. Another way to check the heater core is through its hoses. There should be two hoses that are connected to the core and the engine's firewall. The two hoses should be warm to the touch while the engine and the heater are both running. If either of them is cold, this could be a sign that the core is malfunctioning.
  
  

Flushing

• The easiest way to unplug a heater core is by flushing it. First, wait for the two heater hoses to cool down before disconnecting them from the engine's firewall. Do this by unscrewing their clamps. Be on the lookout for coolant leaking out. Use a garden hose to pick through an opening. Turn the water on and check where it leaks out. Be careful with the hose. Too much pressure can cause damage to the heater core. A steady flow of water is good. If the water coming out is clear, connect the hoses to check if the heater has been unclogged.
  
  

Replacement

• If after using the garden hose the heater core remains to be clogged, you will have to replace it. The exact procedure to replace and install a heater core depends on the car model, but it usually involves a complex set of steps. It involves draining the coolant out of the system using drain plugs, disconnecting the hoses from the engine's firewall, removing the car's instrument panel, disconnecting trim panels, uninstalling the steering wheel, electrical connections, air vents and airbags. The heater core is typically encased in a house which you have to open. It would be best to use a manual or guide for your car model, or better yet, hire a professional to replace the heater core for you.