Teutonic efficiency at its finest. Why add another switch to the dashboard? You only need clean headlights when they're on, right? And logically, you only need to clean them when the windshield is dirty, too, right? Or at least that's what I suspect the engineers thought processes were.
This info is in the owner's manual. If you don't have one, call 1-800-FOR MERC and they will be delighted to sell you one!