The control cable from the head unit should go directly to the Heater Control Valve, located under the hood, usually on the passenger side. The heater hoses from the intake go to this valve, and the cable opens and closes it, in order to let hot coolant circulate. If you disconnect the cable from the valve, you should be able to close the valve manualy, in the normal direction the cable would closed it. If the valve is binding and hard to move, this may explain why the cable is missing, it may have ruined the previous cable, and someone removed it. If this is the case you'll need to replace the valve before you can shut the heat off, or bypass the valve - but this isn't worth it if you plan on using heat in the future.
-Rip
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