Just because they're intact doesn't mean they're working. A thin layer of corrosion between the electrical fitting and the body of the car is all that 's require to muck things up.
It's a bit inconvenient to clean up these grounds - the headlight assembly has to be removed. Here's a quick way to look for the problem. Go get a spray can of electrical contact cleaner - I buy mine at Home Depot. Give one of these grounds a good shot of cleaner. Then go turn on the headlights - see if the wiper goes. If you find a ground which stops the wiper - that's your problem. Spraying the ground is a temporary fix - it'll last a day or two - so you'll have to go in and clean it up the hard way.
BTW, the ground for the headlight washer system is the same place I sent you looking. I know from experience that if the headlight washers lose their ground, the windshield wiper will refuse to shut off - though in my case it was on low speed, not intermittent.