Changing the heater core requires complete removal of the dash panel and plenum box. To do this you have to remove the steering wheel, steering column, evacuate and recharge the refrigerant system and drain the radiator. I would only consider changing the heater core if it is physically leaking into the passenger compartment. If the problem is heat transfer to the HVAC system only, the problem is more likely with the blend doors. Check heatertreater.net for pictures and diagnostic information.