I'll tell you what, mine did the same thing on my older 81. After a quarter million miles of rainy weather the edges of pulleys will wear down and the cable slips off. Mine wouldn't stay on. So I had a mechanic replace the pulley with another one (which was used)but it works fine. He did charge me too much.
About $100 to fix it. I now keep a couple extra pulleys just in case my 84 does this. If it ever breaks again I could fix it myself this time. They are kind of hard to get to. Take out the glove box and the heater duct on the pass. side out to inspect it to see how good yours are. The cable wraps around the "oval" pulley which is controlled by the "arm" on the wiper motor. Also check the clip on the wiper motor that controls the arm. Mine was loose and I didn't check it.
A year later it popped off (in a rainstorm of course)I got home slowly and had to put a new clip on it that night. So check yours while you are under there, they wear out too.
Ben