Heating core location

It sits inside the heater box under the dash. Don't even think you're going to replace it yourself unless you have a mentor looking over your shoulder. Get a copy of the manufacturer's service manual and read through the many pages of instructions with line drawings before considering doing this yourself. Other than AC system specialists, most mechanics dread doing this job, and it's even harder on GM vehicles. GM vehicles are designed specifically to go together quickly on the assembly line, with little regard to future serviceability. You'll have to lower the steering column, and pull the dash assembly back. The AC system will need to be discharged and recharged. That requires special equipment and safety equipment. Escaping refrigerant can cause blindness and frostbite. Pumping in the wrong amount of refrigerant can cause unsatisfactory AC system performance, and potentially cause damage to the compressor.