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When its first reports as a new account it *may* drop your score slightly, but if you let it age a few months you should be fine. If you're going to app for the auto loan very soon I would wait.
I would wait if the credit card is not important to you, there's no point in jepordizing an auto loan over a credit card.
I don't think it would jepordized the loan, but it could potentially cost some interest if his scores are straddling a breakpoint, such as 620/680/720/