Where is the thermostat on a 1985 Chevy G20 Van?

The thermostat on a 1985 Chevy G20 van is located under the intake manifold. This is a common location for many vehicles of that era. You'll need to remove various components to access it, including likely the intake manifold itself, making this a job best left to someone familiar with automotive repair. You should consult a repair manual specific to your 1985 Chevy G20's engine (e.g., a Haynes or Chilton manual) for detailed instructions and diagrams.