Like Yeni stated, you would have to say "My car is being repaired now." I would say "now" instead of "at the moment" since most people say "now" instead of "at the moment". "At the moment" it correct English though. I am just using a more popular adverb which means the same thing, which may be why Yeni changed it to "now" in her sentence as well.
Anyhow, to avoid the passive voice though, you can also say, "They are repairing my car now." The pronoun "they" will be understood by everyone to mean the mechanics at the auto repair shop. :)