IRS Form 8300 is a form that a business (the car dealer in this case) must fill out if they receive more than $10,000 in cash in one transaction or in multiple related transactions. The dealer may fill out this form for transactions that are less than $10,000 if they are potentially suspicious.
If we're talking about an $8000 check and $9500 in cash, that wouldn't require the dealer to fill out the form. But it would certainly tend to look suspicious that you were trying to arrange things so that your transaction was just below the mandatory reporting threshold. I'd certainly tell the dealer that it would be prudent to file a Form 8300 for this transaction if they asked me.
You shouldn't care whether the dealer files Form 8300. It's no work for you. And unless you're hiding income from the IRS, it shouldn't raise any red flags. The IRS may investigate if what you're reporting on tax forms and what banks and others are reporting don't make sense (i.e. someone with $30,000 in reported income making multiple $15,000 cash transactions over the course of a year).