i could _almost_ guarantee the pot that controls the settings is wearing out or corroded. My guess is you would have to tear down the steering column and fix the stem.

The motor _could_ be damaged as well. If the brushes are shot it will not be able to kick in at lower power. I might turn the wipers on and take a volt meter to the hood...you know, measure off the wiper motor and see if its being powered when on that first setting. that will tell you if the motor is busted, or if the switch/pot is busted.