ok I have a 1990 f250 sc 4/4 My wipers only work in high speed. I got a new wiper switch(at junk yard) installed it and still only works in high speed. I removed the wiper motor and put power to it and both the high speed and the low speed both work I think it has to be the wiring? Any body have any ideas on how to isolate this problem or can it be coincidental that both switches failed the same....

 

On my 04 F250 my wipers stopped working traced to the multi function switch, changed it and nothing. Turned out to be a carroded wire going to the motor.

 

Had same problem with my ranger. It had a wiper delay module under the dash in the middle by the ash try. I got one at the junk yard. they work great. Part stores dont list this part you can only get it at the ford dealer. Good luck