In my experience Zcon is my favorite sub, a pair on 3.5k will be plenty brutal and yet sound very good playing a wide range of music, strong bottom and still do very good up top even on a 30hz tuned box. I had 3.5k on my pair of 12 zcons and they always amazed me. Zcons have amazing cooling. I could go on for hours and they would barely get warm. The Till this day I own 4 zcons out of 5.
Measure your back seat space to see how much you can actually fit back there