Possible Duplicate:
“User accounts” or “users account?”
What is correct way to use -s when talking about multiple plurals?
For example, what is correct: items positions or item positions or items position? In context: "I will update all items positions".
What is correct cars categories or car categories or cars category? In context: "Please tell me what cars categories do you know?"
In your first example, you can use all the constructs you suggest. In items' positions (you need the apostrophe to make this better) all modifies both nouns. In item positions, all modifies only positions but the meaning stays the same: the items in question have different positions. In items' position, all modifies items, meaning that all the items have the same position.
In your second example, the one that sounds natural English is car categories. Cars' categories doesn't sound ungrammatical, but I'd use the first one. As for cars category, it means that there are many cars but only one category, which doesn't look very likely.
In your examples, the first nouns item and car are modifying the second nouns position and category. Typically in this kind of construction, the first noun is singular (e.g., a tooth brush, not *a teeth brush) regardless of the number of things you're talking about. The second noun, which is the head of your noun phrase, can be singular or plural.
