how do i repair intermittent wipers on 2000 ford windstar?

Those controls are not in the wiper unit itself but by replacing the arm control on your steering wheel column. That little twisty knob you use to operate them... that is where you want to look to replace. It is a matter of taking apart the steering column plastic jacket, disconnecting the arm's wiring, possibly the arm itself and then replacing it. Most likely it got gummed up inside there, so it is VERY possible that if you go to the store and get some electrical contact cleaner with a little straw and you shoot that up inside the twisty separation (if there is large enough gap) and then twist the control back and forth several times that it will clean the oxidation and or crud off the terminals inside that unit enough for them to get your wipers to work again.