What is the location of thermostat on a 1999 dodge Dakota 3.9L V6?

The thermostat on a 1999 Dodge Dakota 3.9L V6 is located on the engine, at the back of the engine block, near the passenger side head. It's not easily accessible; you'll likely need to remove some components (potentially hoses and possibly other parts) to get to it. A repair manual specific to your year and model will be very helpful in showing the exact location and removal procedure.