Ok, I've been looking at this for a while now and I think I may have found out what I think is causing the issue.
I took the heater control out of the car yesterday and brought it home and sat down and studied it.
I found out that at each end of the slide that the lever goes across and back on, have small nodules that lock the levers in place (see picture 1).
What I also found out is that the levers have plastic guides that allow the levers to slide evenly along the control (picture 2).
Now, I think through a lot of use (being a California car) the plastic slide and the little nodules have worn sufficiently to not allow the lever to lock into place and turn the switch off (switch picture 1 in previous reply).
What do I plan to do about it, hmmmmmm, not sure.
I did think I could remove the switch all together and remote mount it?
Pull the whole unit apart and swap the plastic slides around until I found one that worked and possibly risk breaking one doing it (age, 50 years).
Buy a whole new unit, anyone have a NOS one for sale?