'The Problem' is ..........not very good design on the cable for the heater
control valve.
To increase heat, they cable Pulls on the valve. No problem, the cable pulls
well.
Gong the other way, reducing heat, the cable has to Push .
You can't push something flexible very well.
There is about a 2 inch piece of inner cable that has to slide back into the
outer cable, pushing the heater valve closed.
Eventually..........instead of the inner cable sliding back into the outer
cable housing.........and pushing the vale shut, it kinks.......
And from then on it can't push the heater valve all the way closed.

VW Fox grade heater control levers and cables - right up to 1991.

Oh, if only that cable got  a shot of we-40 or any half decent spray lube
once or twice  a year !!!  
Takes but a minute to access it too, up by the heater control levers.....
You spray that cable, so those two inches or so of shiny metal solid cable
can slide back in the outer housing !
The cables last fine years even, they just kink from lack of any sort of
attention or lube.
Poor design there is the main factor, that and people or shops never lubing
detail things like this.
So easy to prevent too.

Let's see, a two second shot of WE-40 ..........perhaps 2 cents worth,
Takes .50 seconds to apply, 
That shouldn't be too hard to do.