The heater core is a small heat exchanger located inside the truck, similar to the radiator at the front of the truck. Coolant is circulated from the engine through the heater core and back to the engine. The heater fan blows fresh, outside air through the heater core; the air is heated and sent on to the interior of the truck.
About the only time the heater core will need removal, is for replacement because of clogging or leaking. Thankfully, this doesn't happen too often; removing the heater core can be a major task. Some are easier than others, but all require working in unusual positions inside the truck and fitting tools into very cramped quarters behind the dashboard. In some cases, the dashboard must be removed during the procedure, another major project.
Certain Land Cruiser and 4Runner models may be equipped with an additional heating unit for the rear of the vehicle. These small units contain an additional heater core, fan and blower motor. Coolant hoses or pipes run to either the under-dash heater or the engine piping under the hood. The Land Cruiser unit is located under the right front seat; the 4Runner unit is located at the rear of the center console.