What is the black box under passenger side front seat of 1997 sebring convertible do?

That's likely the electronic control module (ECM), sometimes also called the powertrain control module (PCM) or engine control unit (ECU). In a 1997 Sebring convertible, it's a crucial part of the car's computer system, managing the engine's functions, including fuel injection, ignition timing, and emissions control. While the exact location might vary slightly by model, under the passenger seat is a common spot for this type of module.