Where is the thermostat on a 1999 dodge ram 250 van?

The thermostat on a 1999 Dodge Ram 250 van is located under the intake manifold. Accessing it requires removing the intake manifold, which is a fairly involved process requiring mechanical skills and tools. It's not a simple "pop the hood and there it is" situation. You'll likely need a repair manual specific to your vehicle's engine (e.g., 5.2L, 5.9L, etc.) to guide you through the removal and replacement process.