Pictures of the interior would help.

For the best overall sound you want your speakers aimed in the general direction of the occupants, not low. The higher you place them, the 'fuller' they will sound. Bass is omni-directional, so you can usually mount subs lower without noticing their position.

where did you get the enclosures for the speakers behind the front seats?

I interested as well putting a single sub under the rear seat for now and a new receiver.

I would like to see which subs you used.