Where is the thermostat located on a 1997 cadillac sedan?

The thermostat on a 1997 Cadillac Sedan (assuming you mean a DeVille, Seville, or Fleetwood) is located under the intake manifold. This is a common location for many vehicles.

To access it, you'll need to remove various components, including potentially the intake manifold itself (a significant undertaking requiring mechanical knowledge and tools). It's not a simple DIY job and typically requires a mechanic. A repair manual specific to your Cadillac model year and engine is highly recommended before attempting this.