Worked in a detail shop for a few years....Steel wool and vinegar are both good options. Also, for really stubborn stuffs, like overspray, or sticker residue, razor blades work good too, just be careful around paint and window seals.
Also, only use razor's and steel wool on the outside of the windows. If you use them on the inside, and have any kind of aftermarket tinting on the windows, you will get scratches there. Don't know if it will scratch factory tint or not.
Tony