If you want small look into the kicker solobarics. They're pricey and require a ton of power but they will take up next to no room. The other option I saw in a toyota camry. If you build a skinny sealed enclosure, I mena like 4 inches deep that covers the trunk side of the back seat and mount the sub in the middle with the magnet facing the rear bumper (pointing outside the box since the box is way to skinny to get the sub inside it) that would not take up much space at all plus the box would make for a cool place to mount the amps. I don't know what this would sound like but it looked pretty cool. I was thinking of trying this myself just to see. It's long enough that barced 3/4 inch MDF would be a must
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Portland, OR 1987 240 dl wagon (not so rusty anymore) 'rustbucket'