Your best bet is to:
1. Consult your owner's manual: This is the most reliable source. The diagram should show the relay's position in the fuse box.
2. Locate the underhood fuse/relay box: It's typically located in the engine compartment, often near the battery.
3. Examine the relay box: Look for a diagram on the inside of the cover. This diagram usually shows the location of each relay and fuse. Look for a relay labeled "Fuel Pump" or something similar (sometimes it might be labeled as "EFI" or related to the engine's fuel injection system). If there's no clear label, you'll need to consult a wiring diagram (see step 4).
4. Use a wiring diagram: A wiring diagram specific to your 1991 Mustang 5.0L is your next best option. You can find these online through various automotive repair resources (like Haynes or Chilton manuals), or through forums dedicated to classic Mustangs. These diagrams will show you which relay corresponds to the fuel pump circuit.
Important Note: Before working on any electrical components, disconnect the negative battery cable to prevent accidental shorts or injury.
