Nah, deep bass is actually not that difficult in a car. Everything with a wavelength longer than the interior dimensions simply pressurizes the entire space more or less simultaneously. In a typical car, at frequencies below, say, 80 Hz, you can take a measurement mic and move it all around, and get a pretty uniform reading at a given frequency. This is also the cause of the "cabin gain" effect, where the bass response actually rises at theoretically 12 dB/oct below this pressurization frequency. I say "theoretically" because there are many factors that will interfere with this 12 dB slope, but for the most part, you do get substantial acoustic gain.
My main problem is tracking down all the shit that starts rattling, heh. It can take some real head-scratching to deal with some of that. I know that automotive engineers these days do spend time trying to minimize this stuff, but they only have to worry about the factory audio system - 16 Hz is out of their jurisdiction.