On a 1999 Chrysler Sebring convertible, the starter is located on the
passenger side of the engine, generally towards the
bottom and slightly towards the rear of the engine block. Access is usually somewhat difficult and may require removing some components for better reach. You'll likely need to work underneath the vehicle to get a good look and reach it.
Keep in mind that the exact location can vary slightly depending on the specific engine (2.5L or 2.7L). A repair manual specific to your vehicle's year and engine would give the most precise location.