The biggest thing to watch out for is them insuring the loan for disability or death. Don't do it. Don't pay for rustproofing, either. Get one of those new car buying guided before you go. They give you all sorts of tips on stuff they try to charge you for and can't. One is an advertising fee. Why the hell would anyone need to pay their advertising costs?
I know you're buying a Mustang, but for those buying other cars, if there is a factory rebate, do not allow them to factor that into the price. It doesn't come out of their pocket so it shouldn't be their concern. You negotiate based on what you're willing to pay over invoice without that rebate. The rebate may put you under invoice.
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351W 89 Mustang GT Convertible