How do you replace thermostat on 1999 Cadillac Catera?

To replace the thermostat on a 1999 Cadillac Catera, start by disconnecting the negative battery terminal for safety. Drain the coolant from the radiator and remove the upper radiator hose connected to the thermostat housing. Next, unbolt the thermostat housing and remove the old thermostat, then clean the mating surfaces and install the new thermostat with the correct orientation. Finally, reassemble everything, refill the coolant, and reconnect the battery.