Where is the thermostat located and how do you replace it on a 1999 Nissan Altima?

Follow the upper radiator hose back to the engine block. There will be a cover plate with bolts holding it to a housing. This housing contains the thermostate. It has to be taken apart with the 2 possibly 3 bolts and a gasket. A LOT of fluid will pour out when you break the seal and open it. It maybe very difficult to get out without special tools to get to the bolts. You will then need to reseal it with rtv or supplied gaskets. I prefer ultra copper rtv.