Here is an interior view of the early doors so you can see how it all interacts. The handle square stock twists on the end over time locking it in on new corners and often needs some persuasion to let loose. Difficulty is there is nothing to pry against without messing up the door skin. It may come down to sourcing a new handle if you have to cut the square? Maybe someone else has a better solution?
https://www.flickr.com/photos/advance-design_parts_co/14184302391/in/set-72157631868452754Also here is a link to the cab manual filmstrip which could help.
https://www.flickr.com/photos/advance-design_parts_co/sets/72157641244124604/
