Where is thermostat on 1999 dodge cummins diesel?

The 1999 Dodge Cummins diesel doesn't have a thermostat in the same way a passenger car does. Instead of a single thermostat in the engine block, it uses a thermostatically controlled coolant bypass. This is integrated into the engine's water pump housing. You won't find a separate housing or easily accessible thermostat like you would on a typical car. To replace it, you'll need to work on the water pump assembly itself.