Hi John,
As Keoke pointed out, it's all about how you route the cable. You've got the demister tubes in the way. Then do you route it under or over the windscreen wiper box's long tube. Then when you get to the clamp on the heater box, make sure it's in the fully open or closed position when you attach the cable as it applies to the open or closed position on the dash side. Sometimes the cable will get twisted and that makes leveling the writing on the knob off. Oh the fun goes on and on. But like most things with the Healey, patience and persistence will prevail.
The trick.....no trick, no treat either.
Good luck,
Roger