re: steel wool on the hardness scale it is softer than glass. Some alloys can be harder. I'd pass using steel wool on old irreplaceable auto glass, but on a newer car 0000 steelwool (softer alloy) or even any grade of brass wool can be used. The problem is if there is other contaminates on the glass (concrete at new home construction i.e. patio doors) get scratched all the time not by the steel wool, but the aggregate in the concrete being cleaned off. A razor blade is the preferred method there.
The o.p. having a contaminate on the glass does not really need a harsh method. CLean, then the product I mentioned (iirc it's ground sea shells - calcium) is softer than glass and will clean it fine.