Have you been in contact with her school? Many of these subjects are addressed in special ed classes at their school, and if you can hook up with those resources, you'll be able to reinforce the messages at home.
My son has zero fear of strangers as well, so I feel for you. He has always been in a 'safe' environment and I have debated how or if to shock him out of that by setting up observers that know him, but that he does not know and letting him actually run away from me at the mall (which he would do if given the chance.) The observers would shadow him to be sure he doesn't get in trouble, or if he does, to make sure it is the kind of trouble that would make a beneficial impression on him. I am not above engineering or 'stage managing' a crisis like this to teach him these kinds of lessons in a controlled manner, rather than having the real horror of abduction/abuse/etc happen to him and disable or scar him to the point of PTSD for the rest of his life.
M