There's absolutely no reason that you can't put a sub in the doors, as long as the sub is made for a free air application. I've heard a couple of cars with door mounted subs and they sounded great. Just don't expect the output of a pair of 15's and be prepared to work on the door quite a bit, because if the subs are capable of moving much air, a lot of vibration will come with it, meaning you'll need to thoroughly damp your doors with something like Dynamat, Raammat or BrownBread material to keep them from buzzing and rattling horribly.