If it is necessary to remove the heater assembly, the cooling system must be drained before removing the heater core.
When a heater core leaks, a new heater core is installed or the old one repaired.
* The heater housing is usually under the dash and must be removed to gain access to the heater core.
* Procedures for replacing the heater core vary with the year, make, and model of the car It therefore necessary to consult the manufacturer's repair manuals for the proper procedure for replacement.
The heater housing is disassembled to get to the heater core.
* Remove the access panels(s) or the split heater/air conditioning case to gain acess to the heater core.
* Remove the heater coolant hoses.
* Remove the cable and/or vacuum control lines (if equipped).
* Remove the heater core securing brackets and/or clamps.
* Lift the core from the case. Do not use force. Take care not to damage the fins of the heater core when removing.
* To reinstall the heater core, reverse the removal steps.
When the heater core leaks and must be repaired or replaced, it is a very difficult and time-consuming job primarily because of the core's location deep within the firewall of the car. For this reason always leak test a replacement heater core before installation.
I have included a picture diagram above. Good luck.