I'd also recommend to just not worry about it too much. I've been through the "audit-by-mail" process twice. Once for my mistake, so I just cut them a check. A second time, it was their mistake - I sent them the documentation I had on a credit I had taken that they didn't think I qualified for. A month or two later, they replied with another possible error from the same return, so I sent them more documentation, and then they closed the case and agreed that my return was correct as filed. So send them whatever documentation you have regarding your purchase of the car, and its eligibility for the credit. They'll either accept it, or ask for more documentation. Nothing to freak out over. In general, the IRS knows what a "normal" tax return looks like, so when you do something unusual like take a large credit, they're more likely to follow up and ask for extra documentation to ensure that you're not cheating.
Wow..!!! Am impressed by your response to this gentleman's problem; a stranger using their expertise, going out-of-their way to assist someone that appears to be at-their-wits-end.... KUDOS
I appreciate the support. The way I see it, I've reaped benefits from these forums far in excess of any I've offered in return. If, for a half hour of my time I can close that gap a little, there's no excuse to sit idle.