Thanks for the thoughtful responses to my original post. Please note the title is heat door not working, rather than anything to do with the various vents.

The amount of hot air seems to be regulated by a motor attached to a rod which opens and closes a door in the airway, located behind the glove box. As I said the motor seems to have failed and the heat door is closed.

The solution I found was to dis-attach the rod from the motor and open the door. I secured the rod so the door doesn't close and that allows heat to blow and thankfully all my vents work fine

Not the most elegant solution as I will not be able to regulate the amount of heat, but that is better than no heat. I'll probably close the door when the summer returns.
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89 240 wagon, 94 940, 300K, 94 940, 141K