All of my subs have always been placed in the trunk. Many reasons for that, but mostly:
1. Almost all decent subs require at least 1 cu foot of space, usually more.
2. Bass is omnidirectional, and below 80hz, there is no "detail" in the bass that you could decipher from your ears anyway. Behind you is absolutely fine.
3. The trunk acts as a bandpass box in it's own right, allowing additional gain from the sub.
Anyway, my reply is mainly an execuse to post a variety of pics I just snapped. Seeing dysfunction26's reminded me I haven't snapped a photo of my car in a while (it is just sitting forlorn in the garage these days);
First, my dual subs in my honda. My custom made box, with 8812 Eclipse 12" subs in them.
The trick to packaging is finding a useful place to dump all the electronics. I mounted mine right to the sub itself, and left the rest just kinda floating on the floor behind the seat.
The honda system is bass-only, with everything else being stock. Mainly just a bass-head ride.
And of course my other system in the mustang. This one was based around SQ. Infinity separates up front, polk in the rear, and an SVS sub in the trunk.
Another round of ghetto wiring. It'd set up this way so I can pull the sub and all electronics for track day. Nothing is hardwird, I used a 240V plug to create a quick-disconnect for the power/ ground / signal lead for the amp, and everything is just bolt in. Taking it out is good fro about 1/10 a sec.
Another picture. In another thread I mentioned a parts express box. This is that $30 box. It's awesome, fit's the SVS sub perfectly.
And when the factory cover is over, all hidden anway. I had to re-install part of the rear seats (the car was gutted for race duty).
Old School alpine head unit from 95.
And of course I can't rave about Optima yellow tops without showing mine:
And a handy excuse to show off the polished chrome.