How about your car says in a middle aged women's nice and calm voice but highly amplified. Kinda like those messages in Harry Potter than read themselves in the voice of whoever wrote it. I would choose my old American History teacher for the role.

"Hello, I have come to find that you would like to steal this car, but I cannot let you. For I have promised to be faithful and only let my owner drive me, and you are definitely not my owner.

Thank you for not stealing me and have a nice day or night, I can't tell which it is since my day/night sensor broke.

(Pause 5 seconds) You know what, I really should gett that fixed, I need find some way of telling my owner, I think he would gladly replace my sensor as he is a very nice guy. Could you tell him? I'm sure he is not far from here by now?" (Car waits for reply)