Where is the radiator drain plug on a 1999 mustang?

The radiator drain plug on a 1999 Ford Mustang is located at the bottom of the radiator, on the driver's side. It's usually a small, typically a plastic or metal plug. You might need a wrench, possibly a small size, to loosen it. However, it's always best to check a repair manual specific to your 1999 Mustang year and engine type (e.g., 3.8L V6, 4.6L V8) for the exact location and size of the plug as it might vary slightly.