Loud low bass in a convertible?

[33hz]

I am curious how one would get loud and low in a convertible. This is not something I am trying for myself more just curious after witnessing an old school chevy convertible drive by with low and loud bass. I mean we heard him like a good 10 seconds before he drove by @ 40-50 mph and his subs were hitting all the frequencies and with no rattles either (possibility bandpass is through the back seat?) I always thought you couldn't do much with a convertible so needless to say I'm very curious how this is done.