Anyone know how I could go about adding a pioneer slim subwoofer under one of the seats ? Could I piggy back off the rear speakers ? When I die, I want to go peacefully like my Grandfather did, in his sleep -- not screaming, like the passengers in his car

nah, a sub will need its own feed.. even if you have something like a 6x4, an amp will do the business for you.

Connecting an amp requires its own power cable from the battery, then you plug from headunit into amp, and into speakers from amp.

what about an active sub ? When I die, I want to go peacefully like my Grandfather did, in his sleep -- not screaming, like the passengers in his car