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 due to 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.