On mine, which I replaced a while back myself and know its right because the heater core had a IN & OUT. Its the right side on the intake, thats IN and the water pump takes the OUT or left. Drivers side is left, pass. is right.
If your AC is not working, I would replace the heater core if it looks like its never been changed. A new one works much better and the old one will leak one day. Thats no fun smelling anti-freeze while driving and finding your carpet soak