Could be the switch itself.... my exprince with that.. wipers wouldnt shut off... then the intrmidinte black box that parks and delays the wipers caught of fire under the dash and it was scary lol i was driving and all of a sudden the truck died and i had a dead short at a in-line fuse wire type burned up by the battery coming off the starter selinoid.... i went to shut my wipers off and the switch fell apart then the black box caught fire all in about 15 secs.... so start with the switch then look at the intermidate box.... it is a soilid state control....
It's the switch. Mine didn't work when I bought the truck, found the motor was bad, replaced it, and my wipers will stop mid-wipe if I turn the switch off. Still need to fix it, just hasn't been an issue with the drought. I just got really good at timing when to turn them off.
For some reason the system won`t let me link you to the info, so I will try to explain what to do.
Go to the `87-96 F-Series page and open it, at the top of the page click on "Tech Info" and when it opens click on "No Cost Intermittant Wiper Fix".