Here's why it's difficult to give a precise location without more information:
* Multiple Fuse Boxes: 1999 Mustangs might have one or two fuse boxes under the hood.
* Relay Variations: The exact relay might be labeled differently depending on the manufacturer of the components used in your particular vehicle. It might be labeled "A/C," "Compressor," or something similar, or it might not be explicitly labeled. Some relays are even integrated into other modules.
* Wiring Diagram Necessity: The most reliable way to find it is to consult a wiring diagram for your specific year, engine, and options.
To find it, I recommend these steps:
1. Consult Your Owner's Manual: The owner's manual should have a fuse box diagram showing the location and function of relays.
2. Locate the Under-Hood Fuse Boxes: Open your hood and visually inspect for fuse boxes. They usually have covers with diagrams.
3. Use an Online Wiring Diagram: Search online for a "1999 Ford Mustang wiring diagram" along with your engine type (e.g., "4.6L V8"). Many websites (some require payment) provide these diagrams.
4. Look for the A/C Compressor Relay: Once you have the diagram, find the circuit related to the A/C compressor. The relay controlling the compressor is usually the key component.
Remember safety precautions when working near electrical components: disconnect the battery's negative terminal before working on the fuse boxes or relays.
