To find it:
1. Open the hood.
2. Locate the Power Distribution Box (PDB). This is usually a large black plastic box, often near the battery.
3. Look for a diagram on the inside of the cover. The diagram will show the location of various relays and fuses. Find the diagram and locate the fuel pump relay. The diagram will likely use symbols or abbreviations; the fuel pump relay is often labeled "FP" or something similar.
4. Identify the relay based on its size and markings. The fuel pump relay is usually a standard automotive relay, not an unusually large or small one.
Important Note: There's no single definitive location without looking at your vehicle's specific PDB diagram. The diagram inside the PDB cover is your best resource. If you can't find it, a repair manual specific to your year and model Mountaineer will be invaluable.