Demister fan

[will105]

Hi my 15 Seat Altea 1.9tdi , now a veteran, continues to limp on as a second car , demister fan just like last winter starts to blow but then a flap closes preventing air reaching vents , think flap is related to preventing exhaust fumes entering cabin while in reverse “ I think “ , took out glove box etc last year and didn’t really do anything and it corrected it , so didn’t come to the bottom of fault . Any advice welcome before I take it apart again .