Does she know that you desire this?
If no, tell her.
If yes, what is her objection to telling her friends? Perhaps she carries some of your shame. She may not want them to know about your tiny secret. It may be best to live with her decision.
Let dust collect on the rear of your car. Then some night go out and write in the dust. Don't spell "WASH ME", but instead spell "TINY PENIS". I doubt that anyone will believe that you wrote the message. You can avoid seeing the script and deny that you knew it was there.