Depending on their size, you can put them behind the seats, of even under them.
They're sub woofers, so you don't need to see them to get the effect from them.
They sell sub boxes for trucks that you can put the subwoofer in. But is better to get one already made. Like this one: http://funadvice.com/r/3k3gkba2lg
these guys have made it through the same dilemma many times and can give you some good advice. [link removed]