The thermostat housing is located at the engine end of the lower radiator hose.
Follow the top radiator hose as it connects to the front of the engine. The thermostat is located in that housing.
inside the radiator hose
1990 Camry 4 cylinder thermostat is located off the bottom intake hose connected to the engine block.
bottom radiator hose
The coolant fan switch is located close to the thermostat.
Follow the bottom radiator hose to where it meets the block.There is the thermostat housing
If you follow the lower radiator hose to the engine block, it will lead you towards the thermostat housing (inside there is the thermostat),
follow the radaitor hose from the top right side it goes directly to the thermostat housing remove the bolt then there is your thermostat
follow the lower radiator hose to the engine. the hose attaches to the thermostat housing.
follow the upper radiator hose other end is connected to thermostat housing
The thermostat is located on the front of the engine under the elbow at the end of the radiator hose