How do you replace the thermostat on a 1991 Cadillac DeVille and where is it located?

To replace the thermostat on a 1991 Cadillac DeVille, first, ensure the engine is cool and disconnect the negative battery cable. The thermostat is located at the engine end of the upper radiator hose, where it connects to the engine. Remove the hose and the thermostat housing bolts, then take out the old thermostat, clean the mating surfaces, and install the new thermostat with a new gasket. Finally, reassemble the housing, reconnect the hose, and refill the coolant.