Where is the fuel cut off switch for a 1999 mustang?

There isn't a readily accessible, easily-located "fuel cut-off switch" like you might find on some race cars or heavy machinery on a 1999 Mustang. The fuel system is controlled electronically, and shutting off the fuel involves either turning off the ignition or, in some rare circumstances, a fuse or relay related to the fuel pump. There's no single switch specifically designed for this purpose.