I'm pretty sure it goes in the hole in the little lever arm just above the spring. You'll have to load the spring a bit to get it in there. It goes in from behind. It's to help assist the closing of the defrost door if I remember correctly.
In my 1966 Dodge Monaco/Polara service manual, on page 24-37, in figure 46 you can clearly see the spring goes into the hole closest to the pivot of Lever C - in your picture that's the hole just above and to the left of the spring as it's hanging down.