You didn't mention which engine you have so this should help you: on all gas engines EXCEPT the 4.7L V-8 the thermostat is located in the upper radiator hose at the engine block. On the 4.7 it is located on the lower one.
Make sure engine is cool so that hot coolant won't burn you. If you have a 4.7L engine you'll have to drain all the coolant from the radiator first.
Remove the hose from the housing at the block, remove the housing bolts and the thermostat will come out. Note the direction of the spring on the thermostat - it should face toward the block. Clean and replace gasket material taking care not to let any old residue fall inside the block. Replace the housing and then the hose. If the hose clamp is the original one you'll probably have to replace that clamp with one that tightens with a driver or screwdriver.
That should take care of it.
In the oil pan
Follow upper radiator hose to the engine, it is under that housing.
On a Jeep Grand Cherokee v6, the thermostat is located in the thermostat housing. The housing can be easily located by following the upper radiator hose to it.
The thermostat on a 1996 Jeep Cherokee is under the housing where the upper radiator hose attaches to the engine.
It is on the back of the engine, just to the passenger side of the distributor.
On a Cherokee the thermostat is under the housing that the upper radiator hose hooks to. On a Grand Cherokee with the 4.7L it is under the lower hose attachment housing.
4.0L, it is where the upper radiator hose hooks to the engine. 4.7L, it is where the lower radiator hose hooks to the engine.
The thermostat is under the housing where the upper radiator hose attaches to the engine.
It should be mounted inside on the back drivers side wall. If you can't find it then you don't have one.
http://wiki.answers.com/Q/Where_is_the_thermostat_located_on_a_1989_jeep_cherokee"
On the 4.7L the thermostat is located under the housing that the lower radiator hose hooks to on the engine.
Its a small dark colored plastic half-sphere on the top of your dash not far from the windshield.