Where is the thermostat on a 1990 geo prizm?

Just follow your top radiator hose from the radiator to where it attaches to the engine. This is your thermostat housing. Drain the radiator, take the hose clamp loose, remove the two 10 mm nuts from the housing and GENTLE tap the housing. This will expose the thermostat to you. Make sure you get a new gasket along with the new thermostat.