On a 1990 Dodge Ram Van (assuming you mean a Ram Van, as Dodge didn't solely use "Dodge Van" as a model name in 1990), the thermostat is located
in the engine's water pump housing. You'll need to access it from the front of the engine, likely requiring removal of some shrouding or other components to get a good view and access.
It's not always easy to spot; you might need a shop manual specific to your van's engine (3.9L, 5.2L, 5.9L are common for that year) to pinpoint the exact location and provide detailed instructions for removal and replacement. A Haynes or Chilton manual for your specific year and engine would be helpful.
