YA or possibly adult novel, mental hospital, red car

Trying to help out a library patron looking for a book she read when she was young. She's currently 34, but doesn't know when the book was published. It had characters, possibly teens, who were in a mental hospital or institution of some sort. They see what they think is a red car down the road and become obsessed with it. They sneak out to examine the car up close and realize it isn't real, but made of wood or cardboard. They take it apart and put it on a roof. That's all she remembers.

Does this sound familiar to anyone?

I think this is The bumblebee flies anyway by Robert Cormier. The plot and setting match my memories of the book, and it would definitely have been around when your patron was young.

Yes, she said that was it. Thanks a lot!

You're welcome!