Not sure about Windex activating anything, or making it stick, what it does do is allow you a chance to either move decals, or squeege out air bubbles. Think about the mechanics. If you stick two pieces of glued material together, they adhere and there is no chance of getting any wrinkles or bubbles out becasue the surfaces are simply too adhered together. If you slip a non destructive medium (soapy water or a window cleaner type item) into the mix, the adhesive can't get any purchase until the medium is gone. Either by forcing it out, or allowing it to dry. If you watch sign companys place the big decals on the sides of delivery vans, they always use some sort of medium so they can slide those big decals around and get them lined up perfectly.

Tom