On my '87, I had the same features as yours with just the Mist, Low, and Hi settings. With mine, I had to replace the whole wiper and turn signal arms and frame, it is all one unit. This required removing the steering wheel top and bottom covers and then the steering wheel itself.

With the latter trucks, like mine, the relay is built into the electronics of the stalk and frame, so it was plug-and-play. However, I believe with your model year, the relay is separate box that is in the passenger foot well cover. The box is green, about 4"x4"x1-1/2" in size, something like that.

 

I swapped mine with a later model intermittent wiper at the stalk and all was good. just plug and play. The only thing different was the plastic clips holding the harness in place under the ignition.