To find it, you'll likely need to:
1. Remove the lower dash panel: This usually involves unscrewing some screws and possibly unclipping some fasteners.
2. Locate the fuse box/relay center: It's a fairly large plastic box.
3. Consult your owner's manual or a wiring diagram: This is the most reliable way to pinpoint the exact location of the flasher relay within the fuse box. The diagram will show the relay's position and sometimes even its physical appearance.
Without a diagram specific to your 1994 Mustang, I can't give a more precise location. Accessing the area safely requires some mechanical aptitude; if you're not comfortable working on your car's electrical system, it's best to take it to a mechanic.