thermostat

If your engine is a 1mz-fe, it is located in back of the intake manifold (drivers side), above the transaxle. Follow the radiator hose that goes under the cruise control to the engine. From radiator, rubber hose, water inlet pipe, thermostat housing, and then thermostat.

You'll be best off with a repair manual, with steps to access and diagrams: http://bit.ly/mitchell_repair_manuals_online