Where does the rear O2 sensor plug into on 93 Chevy z71 truck?

The rear O2 sensor on a 1993 Chevy Z71 will plug into the wiring harness near the sensor itself, under the truck. There isn't a single, easily identifiable connector like a large block. It's a smaller connector specifically designed for the O2 sensor, usually a two-wire or four-wire connector depending on your specific engine. You'll need to trace the wiring harness from the sensor to find the connector. It will be located relatively close to where the sensor is mounted into the exhaust system.