* Under the hood: This is where you'll likely find the main Powertrain Control Module (PCM), often the largest module and the one responsible for engine management. It's usually attached to the firewall or inner fender.
* Under the dashboard: Various modules related to the body electronics (like the Body Control Module or BCM) are typically located behind the dashboard. Accessing them requires some disassembly.
* Other locations: Smaller modules may be found in other places, such as near the various components they control (e.g., near the transmission for a transmission control module).
To find a specific module, you'd need a wiring diagram for your 1998 Buick Century. These can sometimes be found online (though their accuracy can vary), or through a repair manual specifically for that year and model. A repair manual would be the most reliable source.