Well, you have options....

I think the best option would be to fiberglass in a 12" sub into one of the corners of the trunk closest to the bumper. That way you can still fold down your seats and will have used of at least 90% of your trunk still. You can wrap the enclosure with matching carpet so it just blends in, would be sick!

Another option is to build an enclosure in the spare tire well, which doesn't necessarily mean you'd lose use of your spare tire but you'd have to get real creative with it and do alot of research, measuring, testing, etc. A friend of mine has a Subaru station wagon and his spare is in the cargo area, his sub enclosure is built under the spare. It's an aperiodic enclosure and he still has full use of his spare! Very stealth setup.

You could also go the IB route as well but in order to pull that off nicely you'd have to make sure your rear deck is complete sealed off. That means alot of expanding foam and alot of sound deadener and likely alot of fabrication and modification to the rear deck.

Either way it goes you have your work cut out for you, my personal opinion is the first option.

As for the size of the box you'd need, should you go that route, that's dependent on the manufacturers specs, the size of the sub and the amount of power you're giving it. The amount of power your amp would need to be is determined by the sub you get and it's impedance. First rule of thumb, ignore peak/max ratings, they're worthless! Just find out what the rms rating is of the sub, whether it's dual voice coil or single, and how many ohms each coil is. So if your sub ends up being 500 watts rms and has dual 4 ohm coils, you'll want an amp that can put out 500 watts rms x 1 @ 1ohm (If using a 2ch amp in bridged mode) or @ 2 ohms if using a mono amp. How does that make sense? Because you're gonna want to connect your coils together to ensure that both coils are getting the same signal and the same power at the same time, so you're gonna wire your coils in parallel resulting in a 2 ohm load. So if your amp has only one channel then you'd want to make sure it does 500 watts @ 2 ohms. But if your amp is a 2ch, you'd need to bridge the two channels into one and doing so halves the impedance again so now that 2 ohm load becomes 1 ohm.

But if your sub is 500 watts rms and only has a single 4 ohm voice coil, you're gonna want an amp that can put out 500 watts rms x 1 @ 4 ohms.

Here's a pic of what i'm talking about as far as the box is concerned....

 

i like the idea of the corner spot but i have never fiberglassed before. i had thought about it and sized it up before but im just not sure how i would do with it.is it hard to shape it to fit the corner?

 

I've never fiberglassed before either, but I'm pretty sure you would fiberglass the enclosure while it's in the car so it would fit perfect. That's the whole idea of using fiberglass. You'd obviously lay down a protective layer of something (I don't know what is used) so you don't ruin the trunk liner, and then just build the box in the trunk.

 

My fiberglassing experience is limited to using it as reinforcement for my amp rack, worked like a charm too! There are tons of tutorials online, spend some time looking in to it and I bet you'd end up doing it that way cause it's really not that hard....especially since you're gonna wrap it and not paint it. If you were gonna paint it that would be another story cause then you're gonna have to perfect the surface to ensure no imperfections show up.

 

The fiberglassed box in the corner would be your best bet and go with a small 8" or 10" w3 i have 2 10" W3's in my trunk and they dont take up that much space and they sound great you wouldnt need more than a 600 watt amp and since the amp is so small you can just hide it somewhere where it wont overheat but i would put it in the corner

 

so i have been looking into subs and amps to finally pick and am just curious (as i have never installed one) if this amp:
Two Channel Bridgeable Amplification
Tri-way Configuration:1/ 2 Channels
2 x 50 Watts Total Output @ 4 Ohms
1000 Watts Total System Output @ 4 Ohms
Variable Gain Control
MOSFET Pulse Width Modulated Power Supply
2 Ohm Stereo Stable
Built-In Variable Subwoofer Crossover

will run the following sub? thanks for all the help and advice.
12" Shallow Mount Subwoofer
1500 watts Peak / 400 watts RMS
Frequency Response 20 - 230Hz
Sensitivity 93dB
Impedance: 4 ohm
Voice Coil: Dual 4 Layer, Long Voice Coils

 

It'll power it, but it's not a good choice. Look for a mono (1 channel) amp that does at least 400 watts RMS at 4 ohms for that sub.

 

This!

 

so as long as the rms is 400? what if the peak is higher then the peak of the sub? does matter?

 

Peak ratings are worthless, pay them no mind.