Where is the computer for a 1989 Nissan pickup?

The computer in a 1989 Nissan pickup (likely a D21 Hardbody) is not a separate, easily accessible box like in modern cars. Instead, the Engine Control Unit (ECU), which is the main computer, is typically located under the hood, often near the firewall. It's usually a fairly compact, black box, but its exact location might vary slightly depending on the specific model and trim level. You'll likely need to consult a wiring diagram or a repair manual specific to your truck to pinpoint it precisely.