Where is the thermostat on a 1991 F350 diesel?

The 1991 Ford F350 diesel doesn't have a thermostat in the same way a modern car's climate control system does. Instead, it refers to the engine coolant thermostat. This is located in the engine's water pump housing, generally towards the lower front of the engine. You'll need to consult a repair manual specific to your 1991 F350's engine (e.g., 7.3L IDI or a different engine if applicable) for the precise location and details on accessing it.