Obviously, you will never get her to do something that she truly does not want to do. However, if she is simply nervous, there are probably ways to help her along.
I think the best thing would be to try to bring this up while on an out-of-town vacation, where she can feel comfortable that no one she knows will see her. Perhaps get a hotel room at least 10 floors up, where you could leave the window open and she can get used to an open window during sex. Also, try
this subsection of Literotica, which is the top-rated exhibitionist & voyeur stories. Perhaps you could read some to her while in bed or driving, or have her read them; this could help "break the ice" on the discussion.
Good luck.