I have a '91 F150 with itntermittent wipers that don't work. Checked fuses,ok, checked voltage,ok, checked motor itself,ok, replaced switch,ok,. now i'm after the relay. WHERE THE HECK IS IT!! HELPPP!!

 

Follow the wires from the switch down to the little black box, that's the wiper control module. Remove the box and open it up. Inside you will find a circuit board with many solder joints that are cracked or bad. Resolder the bad connections and reinstall. It should work like brand new.