Why would you ever want that? When I think of car audio, I think of the ghetto blasters that are played so loud that the sound of the plastic car body sounding like it’s gonna vibrate clear off the car frame is louder than the music itself.
If you've ever actually spent time in a car with a proper system and adequate dampening materials installed you hear no rattles or buzzing inside. It's similar to having a massage chair when it's bumping loudly. But you're right about "ghetto blasters" being what I'm referring too. I have a high end high powered sub sitting in my garage that I thought I could use for this.
You guys who don't listen to the type of music that is built around slamming bass will never comprehend why anyone would want this. It'd be stupid to build that style of system to listen to symphonies and jazz. You don't need a bass volume knob for that type of listening :P