The little box with the ducts all connecting to it is the heater control box. It's just a simple box with some moving flaps inside to allow air into any combination of two vents - one low level one to heat the cabin in general, and one above the windscreen to act as a demister.
These are quite hard parts to find now, so I'm glad this one works well enough. The cabin heater control is difficult to turn off, but I'm pretty sure that's an issue with the cable itself rather than the control box.
I'd like to take this off at some point to give it a good clean, lubricate the moving components and maybe give it a lick of paint.
