Where is the rear heater core on 2001 ford expedition?

The 2001 Ford Expedition's rear heater core is located behind the rear seat, within the HVAC system's ductwork under the cargo area floor. It's not easily accessible and requires removing parts of the interior to reach it. You'll likely need to remove some panels and possibly the rear seats to get to it. A repair manual specific to your year and model Expedition will provide detailed instructions and diagrams.