The trip switch isn't working. Maybe it needs lubrication, or a slight adjustment. I'd remove the cover and have a look at the mechanism. I don't have any pictures to help.
Redbird, Tickled shitless that it worked for you too!I disconnected the vacuum hose, used a short piece of hose I had, a few drops of 3-n-1 oil, blew it in, moved the wiper back and forth. Hooked back up to vacuum line, fired up the car, turned on the switch and works great.