What type of glue is it? You will probably have to sand the glue off using something like Squadron's Tri-Grit Polishing Stick (if you don't have one now, you should really think about getting one). After sanding the glue off, use a polish for plastic such as Novus or Tamiya. You can also use Colgate Toothpaste (gels don't work very well), but I find the actual polishes work better for clear plastic. Buff the sanded area. The window will look foggy now. After buffing, make sure the surface is clean and apply as many layers of Future as you want. I recommend using Future for clear parts because it dries slow and is naturally self leveling. Future is an acrylic floor coating which can be bought at any local store. It is placed next to the waxes and cleaners, but it is not a wax.