The fuel rail pressure sensor (FRPS) on a 2004 Ford Freestar is located
on the fuel rail itself. It's a small sensor, usually cylindrical, mounted directly to the top of the fuel rail. The fuel rail is located on the top of the engine, near the intake manifold.
Because of its location, you'll likely need to look closely and possibly use a flashlight to spot it. It may have a wire harness connected to it. A repair manual specific to your vehicle year and engine would provide the most precise location and image.
