My 82 Cherokee Chief does not have an air canister (the black plastic drum), yours may. I just resolved the issue by connecting my heater control vacuum line. Here's how it went in case anybody is interested. My heater controls are vacuum operated with no electric power by three vacuum lines that connect to the heater controls on the instrument panel; one line provides vacuum from the engine through a line entering the firewall (either through the canister or mine was directly from the manifold which caused the confusion); the second line runs from the heater control in the instrument panel to a control valve under the dash on the passenger side that opens when you press the heater button; the third runs from the same heater control to the control valve under the dash behind the glove box that opens when you press the defrost button.
Keep in mind that your blower motor, heater controls, and a/c (if you have it) are independent units. They just work with one another...if that makes sense.