The car broke down

Well, I know this is going to be an obscure one, but once, a long time ago, (in a galaxy far, far away), I was teaching English to a group of engineers who asked me a simple question: what is the subject of 'the car broke down'? Of course, I answered 'the car', but my inquisitive mind couldn't help thinking that, if the definition of 'subject' is 'the responsible for the action', how could we hold 'the car' responsible for breaking down? and if we do, then will is not only inherent to living beings. (I know what Nietzsche would say).
I asked my Greek professor about the Spanish version ("el auto se rompió") and I told her I could only understand the expression if that "se" refered to a passive voice, but she said it wasn't like that. It was only a "se" demanded by the verb. It might make sense from the grammar point of view (?), but it still doesn't answer the problem of will...

"El auto se rompió" IS passive voice Gabriel. That's for sure. "se" for passive voice is as much used as the other passive, sometimes it depends on the sentence, for isntance, "el auto fue roto" sounds so bad it's hardly ever heard, never I'd say.
Now, taking into consideration this sentence is a passive voice, when you analyse it you have "el auto" is the subject of the sentence, so you were right. IT IS the subject of the passive.
Who broke the car? How did it happen? Why it broke down? Nobody cares. That's why the passive voice is used in this sentence, because what just matters is that "el auto se rompió" (and you have to walk:) -just kidding)

Hope it helps!

Break down es un verbo intransitivo, así que la acción del verbo no se transmite al complemento. Es el caso del castellano "muero", donde es muy diferente ser el actor de un verbo transitivo y matarse (pronominalmente), que ser el sujeto de un verbo intransitivo y simplemente morir(se).

En este último caso, como en el de "se rompió", hay un uso pronominal del verbo que normalmente tiene varios sentidos, como distinguir acepciones: ver (la luz), verse (con un problema), dar regalos, dárselas de noble; revertir la acción sobre el sujeto: matarse, romperse; intensificar el significado o darle un toque más personal: "¿le interesa esto realmente a alguien?"; o incluso a veces aparece el pronombre como dativo de interés: ¡se nos casa la nena!, o a veces tan solo como expletivo.

En el caso del automóvil, romperse/break down implica que el propio vehículo es víctima de la acción y no su causa, y el uso pronominal castellano dice que la causa es desconocida o que no viene al caso saberla y sólo nos importa la consecuencia: se partió una biela (causa desconocida) y rompió el bloque del motor (causa conocida) , por lo que el auto dejó de funcionar (consecuencia).

I think "The car" is the subject and doer of the action because the verb "break down" is used metaphorically. It is a very common use in language to personalize objects and animals...I hope this helps!:)