The washer pump on a 1996 Chevy Silverado 1500 (and most likely other 1500 series trucks from that year) is located in the
front of the vehicle, typically behind the bumper, near the passenger side wheel well. It's often tucked up fairly high and might be difficult to see without removing some of the splash shield or other under-body components.
To be certain, you should consult a repair manual specific to your truck's year and model. A diagram showing the location of the washer pump will be immensely helpful. You can usually find these manuals online (often as PDFs) or at auto parts stores.
