Hmmm this is somthing I have wondered too! I cant see the Kanji on this computer, so let me make sure I too have it right!
1. I want a car
(boku wa) kuruma ga hoshiiku kudasai.
2. I dont want a car
(boku wa) kuruma ga hoshikunai.
Right? Thanks for posting this GoldCoinLover, I always wonderded this!