Problem fixed! - cheap and cheerful. Its a bit late to respond that i have solved this problem quite a while ago - but better late than never. Posting in case it can help someone else with the same problem. TO RECAP - hot air was coming into the cab, regardless of the temp setting. It would cost $1,500 or so to get an auto electrician to remove the dashboard to investigate. My solution cost $2.50!! WHAT I DID: I took out the radio and centre console plastic cover. This exposed plastic moulding, behind which was the hot/cold air flap mixer - I knew this because the rod from the temp control entered it through a hole. I used a vibration cutter from my wood-working toolbox to cut a square of the plastic out of the plastic moulding. Then I could see the linkage from the temperature rod to the air door had become dislodged - a pin was out of a hole. So as the temp dial was turned, the door was not being moved. Using two screwdrivers and moderate force, i was able to persuade the pin back into the hole and voila! - it then worked! So what was the $2.50 cost? I bought some epoxy glue to glue back the piece of plastic that I had cut out. Then the centre console cover hides it all.
