I love the Utena movie too. The scene where the rose signet drops out of the roses into Utena's hand is still one of my favorite scenes, and the soundtrack is amazingly kick ass. Do not listen to the tracks of Anthy's escape while you are driving. You will speed.

Anyway, as to the cars, I always took it as having either one of two meanings, based on two points. For one, the movie it titled Adolescence Revolution. The other point is Akio's speech about a car rusting without it's key. I think the point was, that if you cannot escape the things in your past that haunt you, if you do not look to the future for the future, then you will never go anywhere. Utena was Anthy's key to escaping, and thus her vehicle for escaping. Shiori was a car yes, but she wanted to stop Anthy from getting away, had turned into a car just to show she could to, and ended up crashing. It's about breaking out of adolescence and into adult hood, breaking from the chains of the past to put you on the path for who you truly are and were meant to be.

Sorry you are feeling sick too, hope you feel better soon!