Where is the thermostate located on ram 3500 6.7 diesel truck?

The thermostat on a Ram 3500 with the 6.7L Cummins diesel engine is located on the engine block, near the back of the engine and relatively low down. It's not easily accessible without removing some components. You'll likely need to consult a repair manual specific to your truck's year to get a precise location and understand what needs to be removed for access. A picture or diagram in that manual will be invaluable.