Stopping Heat coming through the vents.

[AmadeusCharger500]

I have a 74 charger that sends heat through the vents even when then Heat register is turned to off. This is a cable style not a vacuum activated. Is this caused by a failure of the valve system that its connected to, out in the passenger side engine bay (inner fender)? and if so is it possible to find a quality replacement?  I allready replaced it with one from a local yard.