The radiator was likely not the cause of the overheating. If the engine is a 2.5L you most likely have a head gasket problem. If the engine is a 2.2L the overheating is likely due to the car simply not being bled correctly.
Either way the thermostat is located underneath the engine, drivers side, to the front of the engine. There will be a housing with your lower radiator hose coming off of it and bolts that hold it onto the block. Remove the bolts, remove the thermostat and ALL the old gasket material. Make sure you use Scotch Brite or some other gentle scrubbing pad (NOT something too abrasive) and clean up the mating surfaces between the block and the thermostat housing. Before installing the new thermostat throw it in a pot of boiling water and see if it works. A lot of times thermostats are bad right of out the box, so its a good idea to take the 3 minutes and check before installing that the thermostat is infact good. Remove all dust and dirt, reinstall a fresh gasket, reinstall the thermostat, and torque the bolts in sequence to the proper torque (Gonna have to check the Chiltons). Now that the car is back together, make sure to fill and bleed the cooling system EXACTLY how it says in the Chiltons.
If the thermostat housing still leaks after reinstallation, remove the thermostat housing again and find a flat surface. A granite or marble surface works best, but a FLAT steel plate (The thicker the better) will work too. The surface NEEDS to be AS FLAT AS POSSIBLE for this trick to work. Take a sheet of sandpaper and put it on your flat surface. Now take the thermostat housing and place it on the sandpaper with the block mating surface down. Gently, slowly, and with even pressure rub the thermostat housing against the sandpaper in a figure-8 pattern, turning the thermostat housing a little every few figure-8's. Do this for about 3 minutes and rinse it under warm water. Reinstall the thermostat and your resurfaced housing with ANOTHER NEW GASKET and try again.