Why is only one wiper working on a 1987 Chrysler 5th Avenue?

More than likely the nylon bushing has failed that connects the linkage rods together. It is a one dollar part that is designed to allow for slippage when the wiper blades are frozen to the windshield. Over time the nylon deteriorates and will eventually fail. They are a bear to get to and even harder to replace. While they can be changed with a big screwdriver/prybar and/or channellock or vise grip pliers I suggest checking with a local Chrysler/Dodge/Plymouth dealer as they have a special tool which pops them into place. Most dealers will only charge you a few bucks to do this and will save your fingers, blood, and curse Words for another day.