Re: 1999 Toyota Sienna LE thermostat location?
Buy a Toyota thermostat, and replace them at around 100,000 miles or 5 years. Brand X may or may not be as durable, and there is such a thing as false economy. Some say the Toyota thermostats never need replaced, but I talked to an engine rebuilder who says he sometimes gets Toyota heads to rebuild or replace, somewhere around 150,000 miles, because of bad thermostats.
For a number of reasons, it is not always a good idea to wait until a part fails to replace it, and a thermostat is the best example I can think of, because you just blew $2,000 in repairs when that sucker breaks.
Keep good coolant in the cooling system. It won't break you.