Where is oil sending unit 2004 silverado 4.3 V6?

The oil sending unit on a 2004 Silverado 4.3L V6 is located at the bottom of the oil pan. It's not easily accessible; you'll likely need to remove the oil pan to replace it. It's a small sensor, often with one or two wires connected. You might find it easier to locate by consulting a repair manual specific to your year and engine. A picture diagram from a repair manual will be very helpful.