Simon, I have never had that particular issue with my 63. You never know, the doors that the cables control may be stuck. I would start by removing the ends where they hook to the heater box to make sure if it's the doors or cables.
Will do! Thanks for the advice, havn't had a chance to dig into it yet. Car was sitting over in a damp part of Washington State so could be some surface rust formed on the cables or like you say, on the doors themselves creating a jam.