Where is a computer on 1989 Chevy truck?

A 1989 Chevy truck wouldn't have a computer in the same way a modern car does. There's no central processing unit (CPU) like you'd find in a laptop. However, it would have several electronic control modules (ECMs), sometimes called Engine Control Units (ECUs), which are essentially simpler, dedicated computers. These would be located somewhere under the hood, often near the engine or firewall. They control various aspects of the engine's operation, like fuel injection and ignition timing. They are not general-purpose computers, though.