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Explanation: Please note that the correct German spelling would be
verrückt wie ein Fuchs (nouns are capitalized, the u in verrückt is umlauted)
Also, the translation you chose is a literal translation that has little or no meaning in German. Crazy like a fox implies that the person did something smart and quite daring, possibly outsmarting someone. None of that meaning carries over into "verrückt wie ein Fuchs". So, if you are trying to impress a German by telling him that he's 'verrückt wie ein Fuchs', chances are that he will have no idea what you're saying.
Just be aware that not all idiomatic expressions have exact equivalents, and idioms and proverbs are especially difficult to render accurately in another language.