There isn't a single 100-amp fuse in a 1997 Nissan pickup truck's standard electrical system. Instead, the power distribution is handled by multiple fuses and circuit breakers of varying amperages, located in several fuse boxes. You'll likely find several fuse boxes: one under the hood and at least one inside the cabin (often under the dashboard).
To find the source of a power problem, you need to consult your owner's manual. It will have a fuse box diagram showing the location of the boxes and the amperage of each fuse and what circuits they protect. Without knowing the specific electrical problem you're troubleshooting, it's impossible to point to a single "main" fuse.
