Yes, I believe you have to take the dash apart to access the vent system. Had this problem with my '76 S III, I had to clean up all the pivot points and grease, I replaced the cable on this, as well as the temp control (replaced both control cables with generic lawn mover throttle cables from the hardware store, cost: $18).. Find and download the 'RAVE Repair Manual' on line, it will be worth it's weight in gold, and it's free.

'99 Disco II
'95 R.R.C. Lwb (Gone...)
'76 Series III Hybrid 109
'70 Rover 3500s