1999-04 Mustang GT (V8), All Cobras, Mach 1, Bullitt: Fuel filter is located on frame rail, behind left rear tire.
1999-04 Mustang V6: The fuel filter on V6 models is located inside fuel tank. Not serviceable - must replace entire fuel pump module for renewal.
2005-10 S197 Mustangs (all): The fuel filter is located in the fuel tank. This means that when you need a new filter, you need to replace the entire sending unit.