Accessing it requires significant disassembly of the dashboard, making this a job best left to experienced mechanics. You'll likely need to remove parts of the glove box and potentially other interior components to reach it. There are no easily accessible external locations for this valve on a 2003 Explorer.
Before attempting to locate and replace it yourself, it's strongly recommended to consult a repair manual specific to your 2003 Ford Explorer. These manuals provide detailed diagrams and instructions, helping you avoid damage to your vehicle's interior.
Trying to find it without a manual is difficult and risky; you could easily break something else in the process.