To find the amplification circuitry, you'd need to:
* Check the head unit: The most common location. Removing the radio (often requiring special tools and knowledge to avoid damaging the dash) would be necessary to see if the amplifier is part of it.
* Look for a separate amplifier module: Less likely, but some higher-end models *might* have a separate amplifier module tucked away somewhere in the dash, under the seats, or even in the trunk. This would require quite a bit of searching. The wiring harness would be your best clue. Follow the speaker wires from the head unit.
Without a wiring diagram specific to your 1998 Sebring convertible, locating a potential separate amplifier module will be difficult and time-consuming. It's more probable that any amplification is integrated within the radio head unit.