Mikael,
The temp control cable goes over to a cam mounted on top of the heater core unit. That cam controls the flapper door in there. If you take the glove box out, you'll be able to see it behind there. Cable could be kinked or jammed up, or just gummed up with dried crud. Might need to be cleaned and lubricated. If you remove the end of that cable at the dash control unit, check to see if the control knob and slider behind it are moving freely. You should be able to determine what's going fairly quickly. Let us know what you find.
Alan
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