The oil pressure sending unit on a 1999 Chevrolet Blazer with a 4.3L engine is located on the
passenger side of the engine block, near the
front of the engine. It's usually a relatively small unit with one or two wires connected to it. You'll likely see it fairly easily once you've got a good look at the front of the engine. Be careful, as the area around the engine can be hot.
To be absolutely certain, consult a repair manual specific to your 1999 Chevrolet Blazer 4.3L. A visual diagram will be invaluable.
