Your URL looks wrong. I think you used a question mark instead of an amp to separate request variables.
I tried using http://www.reddit.com/search.json?q=ferrari
and I got a lot of results in JSON format.
This will allow you to do the most basic search for reddit posts.
You can add other parameters, described on the reddit API page : http://www.reddit.com/dev/api#GET_search. You can also limit your search to one subreddit.
For example, to get the same results, sort by newest : http://www.reddit.com/search.json?q=ferrari&sort=new