The location of the auto tensioner pulley (or just "tensioner") on a Dodge Dakota varies slightly depending on the engine and year. It's generally part of the serpentine belt system. However, it's almost always
near the front of the engine, on the passenger side (right side as you're facing the engine).
To be precise, you need to specify the year and engine of your Dodge Dakota. A picture of your engine bay would be very helpful. A repair manual specific to your vehicle's year and engine would show the exact location.