i didnt have to hit my knob to get them to work but had to keep jiggling the the knob and i could rotate the knob almost a whole 360. when i pulled the dash console out to repaint it and the dash i was checking out the wiper linkge from the knob to the wiper motor(sure beats standing on your head). i saw that the knob arm was bent and the grommet that holds the linkage to the knob was missing. i removed the linkage and moved the arm on the wiper motor to the on position and low and behold the wipers work as perfect. luckily i have another truck with the same set up and saw that the knob arm is straight and has a slight twist forward to line up with the wiper motor arm. maybe your fix is just as simple. new grommet should be on the porch when i get home tomorrow