I have a 71 Duster and I put in a new heater core because I wasnt getting any heat and now im still getting nothing. It blows plenty of air but its cool (outside temperature) air. Is there something easy that im missing here?

Are both heater core rubber hoses hot after the engines been running? If they are you know hot waters going in and out of the core so the rest you have to look at is the air delivery.

I had same issue in mine. Turned out that the inner metal wire of the cables that activated the doors kinked and the doors weren't opening.

Make sure your doors open and close freely by hand, then check the cables. If yours are vacuum operated then check the lines and that the actuators still hold vacuum.

Operating Cables That control The temperature And Direction(Heat/Defrost) Of The Air Are Pretty Weak. Take A Look At Them, They Are Hiding Up Under The Dash Running Between The control Leavers And Heater Box. Appx 1/4" OD.