Where is the computer on a 1990 Buick Century?

A 1990 Buick Century doesn't have a computer in the way a modern car does. It has a number of electronic control modules (ECMs), sometimes called Engine Control Units (ECUs), but not a single central "computer". These modules are distributed throughout the vehicle, and the main one controlling the engine would likely be located near the engine itself, possibly under the hood, or in the area of the firewall. Other modules might be found in various locations under the dash or elsewhere in the car depending on their function (like for the climate control system).