Jvier,
My guess is that your are referring to the rear door on a '51 panel body.
Have you removed
the lock clip? As I recall it is a curved plate that pulls right out the door and releases the lock mechanism. It is often rusted in place - spray with a lot of WD-40, etc., from the door edge side. There is a 90 degree edge on the clip that you can pry under (and scrape the paint, of course).
Tim
