Where is the thermostat housing on a 2002 buick century custom?

My 2002 Buick Century custom has been overheating after about 5 min of the car being on, I am sure it is the thermostat but where is it.|||If you have the 3100 engine,the thermostat is below the throttle body where the upper radiator hose is going. If you have the 3800 engine,it is toward the front of the car from the throttle body where the upper radiator hose is. One note is,no engine overheats in 5 minutes.The coolant will not get hot enough in 5 minutes to be able to open the thermostat. I would bet you have a head gasket problem. Most likely it would be the rear head gasket pumping cylinder pressure and heat into the coolant.