ほしい is not a verb, but more of an attribute. Unfortunately, the verb Want does not have a straightforward equivalent in Japanese. You will have to know several different forms to say it in different situations.
At beginner stage, just learn to say you want something first.
車が ほしい です。
I want a car.
Note that the subject is the car, not me.
Here the car is the subject, not anymore the object as in the English sentence.
Literally it reads A Car Is Desirable.
You can imagine 私は (to me) is hidden and implied. However, for some deeper reasons, you cannot use this pattern by simply adding XXは to say he wants a car or you want a car.
Again, usage of は has not direct association with whether the term is the subject.
To your question Robb, it is not a matter of は or が。It depends on the context.
If you say 車がほしいです while chatting with your friend in the pub, you mean a generic car; If you win in a TV show and are allowed to choose your gift among a Toyota, a giant LED TV, jewellery, etc. and you say the same 車がほしいです, it would be the specific Toyota in the gifts.
It is not a passive voice statement. Japanese has its own form of passive statement.
Imagine that there is an adjective Hoshii which means "wanted by me". You use this adjective as you would use other adjectives to describe a car, like a car is expensive, a car is necessary, a car is extravagant, or a car is "wanted by me".