Hehe, What you dont realise is that I REALLY know my In car stereo
My steg amp is possibly going to stay... Its a damn good amp, 105watts rms x 4 channels, 2 ohm stable, bridgeable (105x2 & 210x1 or 210x2) and looks really funky.
Problem with what you describe is that I plan on keeping the rear seat itself, I do carry passengers from time to time.... And mounting a sub beneath the seat pointing up will not work as the sound waves cannot get through the material the rear seat is made of without being greatly distorted (makes the bass sound muffled). I have tried this before.
What I have found is a set of 6 inch subs that have 3 inch excursion! Massive magnets and can handle around 175 watts max... They are designed for a 0.1 cubic foot box! Thats TINY. The speaker would fit under the rear seat pointing towards the front of the vehicle (so no seat to get the sound through!) and the box is small enough that I will be able to chuck together a fibreglass enclosure easily (and it keeps the weight down too). I will put together an article on creating a fibreglass sub box for the site sometime too
Its EASY when you know how
Once I have sold my existing stereo I will have a more in depth look into small subs/enclosures that weigh naff all.
I dont like infinite baffel subs (freeair) they never sound quite right...
Cheers for now,
Sam C