Might be I can make an 8" driver work - and I found my amp - just 38mm thick http://www.genesiscaraudio.co.uk/global-en/products/profile-range.php (the Profile 2) This will fit under the seat Done a deal for a second hand one
JL Audio also makes a new "flat" subwoofer...I've personally used the 13" myself (thats what she said) and with the right wattage it sounds amazing and with very little enclosure space.
I have just about solved this for anyone that's interested. I have put an 8" Kicker CompVT (shallow profile) subwoofer into the void at the end of the passenger footwell by replacing the original metal plate with an MDF plate I made. This has a big hole in it & is mounted using the original mounting holes & captive nuts used for the stock plate A second plate that carries the speaker is screwed to the back of that. With shallow mounted woofers the trick they seem to use (to reduce the overall dimension) is that the cone will move beyond the front of the thing so you need to make sure it sits in a recess & there is enough clearance at the front to allow the cone to move freely. There is then a metal grate over the top of the thing. Its very solid & very strong. I have "stolen" about 15mm from the end of the passenger footwell. I've also put some wadding in the void behind the speaker because that's supposed to make it behave as if it were a bigger void - no idea whether that will help or not! The only thing I have had to relocate from the void is the relay box for the Airconditioning relays (that is normally secured to the back of the plate). This may not work on a LHD car but in the RHD car there is a handy space just to the left of (and behind the line of) the clutch pedal directly above the centre consiole. In a LHD car I suppose the equivalent will be beside the accelerator. The relay box fits neatly up in there & doesn't interfere with anything. Unless you stick your head under the dash you really cant see it. "As the crow flies" its physically very close to where it was so the two connection cables are the right length as well so no problems connecting it back up. I miscalculated on the amplifier - the Genesis Profile 2 is about the smallest thing I could find & its still too big to go under the seat. http://audiovisualworld.co.uk/shopping/products.php?product=Genesis-Profile-2-%252d-2-x-75watt-2-channel%2C-bridgeable-into-single-(mono)-channelAmplifier (Got mine 2nd hand off Ebay for less than half the cost of a new one). Dimensions of this are 240x217x38mm so its very small. I will run it "bridged" which means combining the output from the 2 channels into one. Its designed to be able to do that for Subwoofers. Run a s 2 channel amp its supposed to deliver 2x75W RMS. When bridged it will be 1x200W (RMS). This is part of the reason I went for the 8" woofer which will handle that.It would have been much easier to use their 6.5" speaker. The issue with installing the amp under the seat is not so much the height but clearance around the seat adjustment bar which interferes. The solution has therefore been to make a circular board that sits inside the spare wheel & the amp is mounted on that. The board has a rubber edge so as to cushion it & is painted red to match the car. Phono, speaker & amp trigger cables run back through a small existing hole I found in the firewall up high in the LH wheel arch behind the splash panel. These come out right in the corner behind the tweeter in the dash & then run along behind the fuse board to the centre console. The speaker wire just goes straight to the void behind the speaker. I siliconed the cables in to seal the hole & so they won't move. No idea how it sounds because I havn't finished wiring it up yet! The installation is entirely reversible. I would think I could remove it and put it back to stock in about an hour.
