the radiator petcock on a 1999 Buick Regal?

A 1999 Buick Regal doesn't have a radiator petcock in the traditional sense. Older vehicles used a petcock—a valve—to drain the coolant. Modern vehicles, like your 1999 Regal, typically use a drain plug located at the lowest point of the radiator.

To drain the coolant, you'll need to locate this drain plug (usually a bolt or similar fitting) at the bottom of the radiator. You'll likely need a wrench or socket to loosen and remove it. Always be cautious when working with hot coolant. Allow the engine to cool completely before attempting to drain the radiator.