Don't forget that the original cables allow to you push AND pull, it is this action that often fails.
The clamp that secures the cable sheath to the lever bracket allows the sheath to slide through it over time so when you think you are operating the flaps you are in fact doing nothing at all. Occasional lubrication of the cable inner is not a waste of time and making sure the cables are adjusted & secured correctly and doing what they are meant to do would solve the vast majority of heater related problems (a new thermostat and sort out of the cooling system would sort out most of the remainder!).