Where is the thermostat on a 1994 Plymouth yoyager?

Follow the top radiator hose until it meets the engine and it will be connected to a metal tube ( thermostat housing), that tube can be removed and underneath or ( inside ) you will find the thermostat, it is sandwiched between the housing ( tube) and the engine block. Be sure to scrape off all traces of the old gasket before putting in the new one and verify that the thermostat is in correctly spring side towards motor.