Bumper stickers and dealer emblems are usually attached to a car with
glue. The best way to loosen glue is heat, so try heating it up with a
hair dryer. Use a putty knife, with its blade wrapped in duct tape to work
the label up as the glue warms up.

First thing I did to every OB I've owned is scrape the stickers off the
windows, I like the cleaner look. And yes a razor blade works great, also do
it when the glass is warm.