When I did it, I initially made a mono recording with an omnidirectional microphone placed where my head would be when I'm sitting in the driver's seat. I used a boom stand stuck through the window, which was rolled up as far as possible and still accommodate the boom. I played back a CD that I'd made for the purpose that contained first white noise and then a series of 2-second sine waves of incremental frequency from 40 Hz to 500 Hz.
Realizing that I was taking a significant acoustical variable out of the equation, namely my own body, I later repeated the test using a borrowed portable recorder that I could hold at ear level while I sat in the car. As expected, I absorbed a lot of upper mids and high frequencies, but even my 200+ lb. mass had almost no effect on low frequencies.
What I concluded from those experiments was that a car's interior is about the worst possible acoustical space for music. Which paradoxically makes it a good test for mixes, even if it's not a reliable reference: if it sounds good in the car, then it's likely (but not a given) to also translate well to better acoustical environments.
You just don't want to make changes to your music specifically to suit the car's uneven response, because that's a rat-hole you'll never climb out from.
A great deal of attention is given to acoustics by automotive engineers, but isolation is the priority, not music. Isolation means external sound is kept out for a quiet ride, but it also means sound is trapped within the cabin, assuring extreme resonance issues and drastically uneven frequency response.
Speaker placement is all about making the speakers invisible and unobtrusive. That leads to acoustical atrocities such as tweeters in the dashboard pointing up at the windshield, and woofers mounted in doors that are actually resonant chambers designed to give a satisfying "thump" when you close them. Not to mention subwoofers in the trunk that rely on conduction through the car's chassis to be heard.
None of this is conducive to a flat frequency response. Then you've usually got a second pair of speakers in the back with L/R reversed, guaranteeing that no one in the car hears a proper stereo image. One good result is that you'll know right away if your mix is not mono-compatible!
And the problems don't end there. Because very low frequencies are physically impossible to support in a space smaller than a quarter of their wavelengths, they have to be artificially helped by EQ. Most factory-installed car stereos therefore have a built-in boost between 60 and 150 (!) Hz. There is also a built-in compressor, which fortunately many players allow you to switch off but are likely enabled by default. And don't forget those dreadful graphic equalizers, but of course WE are smart enough to not use them, right?