I've seen 4x 12"s behind the seat of an s-10 before, even installed some.

I wouldn't recommend doing it unless you have a good knowledge of the speaker parameters & how to construct a proper enclosure.

In most cases, a single 10" or 12" sub will be more than enough for most people. There should be ample air space for a single or possibly dual 12" subs, as long as you have an appropriately sized enclosure.

If you use subs that require 2cu/ft of air space, you'll be lucky to get one behind the seat, if you find shallow mount subs with less air volume required, you'll be able to install more.

__________________