To locate it, you'll need to:
1. Remove the rear cargo area's interior trim: This will involve removing panels and possibly fasteners near the tailgate to gain access to the rear wiper motor area. A shop manual for your specific year and model will be invaluable here.
2. Locate the rear wiper motor: Once you've removed the trim, the motor should be visible.
3. Inspect the wiper motor housing: The washer pump is usually part of the assembly, often mounted directly to or very near the motor itself.
Without a shop manual specific to your 1996 Chevy Tahoe, accurately pinpointing the location of the pump is difficult. Using a manual is highly recommended, as the location can vary slightly based on trim level or options. A Haynes or Chilton manual for your vehicle would provide detailed diagrams and instructions.