This is a common problem on 116 models - when the sliders become inoperative, it means that either of the two flaps has gone into the 'overcentre' position and become stuck. This invariably means that the foam that's glued to the flaps has disintegrated. It is in fact the foam that prevents the flaps from going overcentre. The first picture illustrates the flap and the poor state of the foam,
View attachment 17046
The two flaps are located within the bottom of the main heater box...