I had the same issue, my heater control was so stiff you'd swaer the cable was completely seized within it's outer.
However, not so!
I dropped the dash, found the little gear wheel, and sinmply softened the hardened grease with a sqirt of WD40.
Working the heater control through its travel a few times with the cable discinnected and rotating the gear wheel in the process freed everything up a treat, I did also flush the cable with some WD40 but 95% of the issue was the hardened grease on the gear wheel.
I finished the job by applying a few drops of ATF to the gear wheel and ran a little down the cable too, with the cable reconnected and the dash back on I re-tested my freshly lubricated heater control and the difference was astonishing, it had gone from 'I think I'm going to break it' stiff, to fingertip light operation forward and back.
Job done
NB: I also used the same hardened grease thinning method on my wiper motor wheel box, and with equally sucessful results.
