The most common fault with the seat heaters is the heating elements, your 2004 Chevrolet Monte Carlo would be no exception. The upper and lower elements work in series so that if one fails they both stop working. The best way to test them is to check the resistance of each heating element separately. If one or both are out of spec the faulty element will need to be replace. This will involve removing the seat cover(s) as necessary to access the element.