Why cougar 1999 overheats?

it could be a few differnet things my 2000 cougar was having the same issue and the best way to do it is start with the cheaper parts first. the cheaper parts tend to need to be replace sooner. first check the coolant level.. if that's fine i would move on to replacing the thermostat (which turned out to be my problem) its a pain to get to because mercury didnt give us much room under the hood to work. and it makes a mess. to replace it its about a 10 dollar part plus some fluid to replace what was lost when changing it. follow the lower radiator hose to the engine the hose should stop at a silver compartment held together by 2 screws. the thermostat is instide there keep in mind all the fluid that is still in the hoses will leak out when you take this appart. as soon as you do this you will see the thermostat pull the old one out then replace it with the new one with a new gasket and use gasket sealer. then refill the collant to the specified level and test it out. next it could be a collant leak water pump.. but i would try the thermnostat first seeing that its cheap and easy to do