The heater core in a Geo/Chevy Metro is located behind its instrument panel. You need to remove the panel, if you want to replace the heater core itself. Like most auto maintenance, you should perform this task with extreme caution.
Unplug the negative cable from the Geo Metro's battery. Raise the car and drain the cooling system (into a clean container, if you plan to reuse it). Do this by opening the drain plug on the bottom of the radiator and then replacing it after all the coolant flows out.
Lower the car and remove the instrument panel (this should take at least two people). Disconnect the heater hoses where they connect to the heater core tubes at the firewall.
Detach the temperature and mode control cables from the heater housing. Take off the housing from the evaporator assembly, unless the car doesn't have air conditioning. In that case, remove the air duct from the housing to the blower motor assembly.
Disassemble the heater case assembly after removing it from the Geo Metro by undoing the bolts and nuts. Once you disconnect the heater case halves, you can remove the heater core from the case assembly.
Install the new heater core into the heater case assembly. Reassemble the case and put it back on the car. Reconnect the evaporator case assembly, mode control, temperature cables and heater hoses to the heater core.
Reinstall the instrument panel. Refill the cooling system (use fresh fluid, if there's any doubt to the old fluid's quality). Reconnect the negative battery cable.
Start the engine and let it reach normal operating temperatures. Check for leaks in the system and make sure the heater is operating properly.
