The rear air blend door on a 2003 Ford Expedition is located
inside the rear HVAC unit, typically under the rear cargo area floor. You won't see it without removing parts of the rear floor and possibly the rear HVAC unit itself. It's not easily accessible.
To access it, you'll need to consult a repair manual specific to your year and model Expedition. These manuals show diagrams and steps for disassembly. Accessing this door requires considerable mechanical skill and patience.
