Removing the heater core in a car is challenging and takes time. Because of the time and labor needed, this a high-cost labor item at a garage compared to the cost of the parts. Most of the cost of replacement will therefore be saved by the do-it-yourselfer. Some mechanical experience and proficiency is needed to be successful.
Make sure the car is cooled from running. Drain the radiator of all coolant by opening the petcock at the bottom of the radiator and loosening the radiator cap so there is no vacuum pressure. The coolant can be saved in a container and reused if it is relatively new, clean, permanent antifreeze, but more likely it needs to be discarded in a safe way at your local landfill/solid waste facility.
Remove the heater core feed lines as they come through the firewall between the passenger and engine compartments. They are the two metal pipes under the hood at the firewall that likely go through the firewall at the evaporator sealing plate. An online technical diagram of your make and model car or a reference Chilton's or Hayne's repair manual can be invaluable here (see Resources). If your vehicle has air conditioning or is a newer vehicle with so little room to gain access that it seems the engine must be removed to access the lines, it is time to rethink the process and take it to a garage.
Access the heater core from inside the passenger compartment on the passenger side under the dash. Remove the top of the dash pad by taking out the screws under the front lip of the pad. Also remove the screws in the defroster vent holes. Because of finish work, screws will be recessed, tucked away and difficult to access. Patience, trial and error and a detailed manual for your particular vehicle with diagrams is again most helpful.
Remove the lower trim panel from under the passenger-side dash if there is one. Look under the dash on the passenger side for the plastic box that contains the heater core. Take out the screws around this box. A screw at the top will be hard to get at, which is why the top of the dash must be taken out so you can use both hands.
Remove the screws that hold the core in place by a steel clamp fastened with screws. First remove the screws; then remove the clamp.
Put a plastic dishpan on the passenger-side floor interior or plastic sheet down to catch any coolant still trapped in the system. Take out the heater core by pulling it gently and wiggling it. Pull slowly, and it should come out fairly easily.
