To find it, you should:
1. Consult your owner's manual: This is the best place to start. It should have a diagram of the fuse and relay boxes showing the location of the blower motor relay.
2. Check both fuse boxes: Carefully examine both the underhood and under-dash fuse boxes. Look for a relay that's labeled "blower motor," "heater," or a similar designation. It might also be a specific number (refer to your owner's manual). Relays are usually square or rectangular and somewhat larger than fuses.
3. Look for a wiring diagram: If you can't find it in the manual, you might find a wiring diagram online (search for "1996 Chevy S10 wiring diagram"). This will show you the location and circuit for the blower motor relay.
Without your specific truck's information, this is the best guidance possible. Be careful when working with electrical components and always disconnect the battery's negative terminal before working on the fuse boxes.