* Within the trunk lid's trim: Look closely at the inside of the trunk lid. There's often a small, removable lens or cover where the bulb is housed. It might be behind a panel or accessed by pressing a clip.
* In the area of the latch mechanism: Sometimes the bulb is positioned near the trunk latch mechanism, but still accessible from the inside of the trunk lid.
You'll likely need a small flathead screwdriver or similar tool to pry open the access panel. Once you locate the access, replacing the bulb is usually simple. It's typically a standard size bulb (often a wedge base type).
If you can't find it after checking these areas, a quick online search for "199X Chevy Cavalier trunk light replacement" (substituting your Cavalier's year for "199X") along with an image search should provide visual assistance. A repair manual for your specific year and model will provide the most accurate information.