Where is the water temp sending unit 2003 Chevy 5.3 v8?

The coolant temperature sensor (CTS) on a 2003 Chevy 5.3L V8 is located on the passenger side of the engine, near the thermostat housing. It's usually a single-wire sensor screwed directly into the engine block or intake manifold. It's a relatively small sensor.

However, there's no "sending unit 2" in the common understanding of GM's 5.3L engine of that year. There's typically one primary coolant temperature sensor that feeds the engine computer (ECM) and gauges. While there might be other temperature sensors for other systems (like the transmission), they are not usually referred to as "sending unit 2".

To be certain, consult your vehicle's repair manual or a wiring diagram. You might be looking for a different sensor altogether, and using the wrong terminology is causing confusion. A picture of the sensor you're looking for would greatly help clarify the situation.