I recently used a product called "Diamondite Spray Clay" on all of the windows of my '00 Sable wagon to remove acid rain spots that were driving me nuts. I found it on the net when I googled "Auto window cleaners" (or something like that).
It comes in a aerosol can and comes out like shaving cream. You apply it to a scrubber sponge that came with it and then rub it on the glass in a circular motion using slight pressure. It worked great and the windows looked like new.
The clay doesn't remove the actual spots because they are etched into the glass. But, it does remove the dirt trapped into the etched area so that you can't see the spots. Then you are supposed to coat it with a protective coating that they offer to seal the spots to keep dirt out of the etched area.
I'm sure this product would remove your marks because it is also supposed to remove paint overspray.
The name on the can is "Diamondite, New Orleans, LA". The phone number is 1-800-426-1380.
Don