I fitted new cables because the old ones were seized, but for some reason the lever was still too stiff to move. The level itself moves, when the cable is disconnected, perfectly well. The cable can be pulled and pushed with a pair of pliers when it's not connected to anything. The flap on the heater box moves perfectly well also, on its own. But when I connected everything, I was at risk of snapping (again) the plastic heater control levers (right where they attach to the metal part of the lever, I'm sure you've seen this thing before, seems most every Bay I come across has snapped and rigged plastic bits).
So I just wired open the flaps for the winter, and it worked fine (probably better, since I could ensure that the flaps were REALLY fully open. Then in one spare set of flaps, I screwed them into the fully open position with some screws through the outer housing, so ALL of the hot air came up to the front. This worked very well, and I just swap out the housings when the season changes.)
Cheers, A.