The camshaft position sensor (CMP sensor) on a 1998 Ford Mustang is located on the
passenger side (right side) of the engine, near the front of the cylinder head. It's usually mounted fairly close to the camshaft itself, but the exact position might vary slightly depending on the specific engine (e.g., 3.8L V6 vs. 4.6L V8). You'll likely find it near the valve cover.
To be certain, you should consult a repair manual specific to your 1998 Mustang's engine. A diagram showing the sensor's location will be invaluable. You can usually find these manuals online or at auto parts stores.