On a 1998 Chevy 1500, the starter solenoid is typically
mounted on the starter motor itself. The starter motor is located on the
passenger side of the engine, near the transmission bellhousing. It's a fairly large, cylindrical component. The solenoid will be a smaller, usually square or rectangular part attached directly to the starter. You'll likely see a large cable going to the battery and another thicker one going to the engine block, both attaching to the starter and/or solenoid.
However, be warned that there's some variation between engine types (e.g., 4.3L, 5.0L, 5.7L). The exact location might vary slightly depending on specific engine configuration and accessories. A repair manual specific to your vehicle's engine and trim level would be the most accurate guide. You can find diagrams online, or better yet, at a local parts store.
