Gmc jimmy coolant problem not sure if its the heater core

[Hawkeyes005]

I have a 2001 GMC Jimmy 4.3L. Its dropping coolant behind the rear tire, i think its running down the fender but im not sure. It doesnt overheat until it loses alot of coolant which takes awhile. It smells like coolant inside and leaves film on windshield making me believe its the heater core. Any help or suggestions would be great.. thank you
p.s i read the other posts and it didnt saying anything about special tools.. i talked to a shop and they mentioned something about refrigerant?