This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
During the late 70's up through the late 80's all American cars had
something called a window shade mechanism to allow for a small amount of slack
to build-up in the shoulder belt. This was to prevent people from complaining
that their belts were too tight. I experienced (ref:June '87 Car and Driver
article by Patrick Bedard) a problem where the seatbelt built up too much
Sometimes the belts, like a windowshade, would never return at all. There is
usually a large plastic button on the 'B' pillar that needs to be fooled into
thinking the door is always opened, which by the way disables the window shade
mechanism and is how the belts return 'home' when you get out of the car.
the plastic button very close to the 'B' pillar, being careful not to cut into
the inner spring
Take a cotter pin and put it through the loops of the spring, this
spring from ever retracting. Chrylser mini-vans are easier in that they
rotating plastic cam with a striker pin that is engaged by the closing door.
Just cut the striker pin and you eliminate the problem.