Obviously, that wasn't what turned him into the mentally ill racist he became, but it's a story told in "Hatemonger," by Jean Guerrero that has stuck in my head -- how Miller complained to friends about how horrible it was -- how he "looked poor" because he was dropped off in that shitty car.
From an Interview with the author in the Los Angeles Times
https://www.latimes.com/entertainment-arts/books/story/2020-08-27/jean-guerrero-hatemonger-stephen-miller-interview
To me the anecdote that best illustrates this is when Miller ended his friendship with Jason Islas because of his Latino heritage, but also around the same time, expressed embarrassment to be seen being dropped off at school by the Latin American housekeeper. It was about being seen as poor, and it coincided with this period in his family life where he had a loss of privilege, moving to a less affluent part of Santa Monica. His dad was getting into all sorts of legal disputes. He was very aware of this change of fortune and wanted to hide it, and he associated his Mexican friend, the Latina housekeeper with it. He wanted to be perceived as elite.
And he learned that if he started to express these really hostile viewpoints, it was a way of standing out and getting attention and getting power, taking back some agency.