Where is the oil pressure sending unit on a 1999 V6 Ford Explorer?

The oil pressure sending unit on a 1999 Ford Explorer V6 is located on the passenger side (right side) of the engine block, near the front. It's typically a small, single-wire sensor. You might need to consult a repair manual or a diagram specific to your engine (a 4.0L SOHC V6 is most common) for the exact location, as there might be slight variations. It's usually fairly accessible once you're under the hood and can see the engine block.