Because the bass is so weak even with the Hi Fi that it seems like its not there.
The funny looking little grills that you think might be vents for air or something at the side of the seat rails next to the window, that's them.
look in this DIY its in the same location
look at the description of the base system in the other articles.
I mean glued in to the underseat enclosures which, look kind of like the ones in the DIY
this is even better look at it