I know exactly what you mean. I have seen just such a device in the Crutchfield catalog. I do not remember whether it was made by Sony or Pioneer, but it was one of those two, I think.

Unfortunately, I saw it in the catalog over two years ago and haven't seen it since. It is possible that it was unpopular and has been discontinued. A sub tucked so deep into the car's spare tire well needs to be pretty large to radiate the sound well, and I don't think 8" is going to cut it for most cars. My guess is that it WAS discontinued for that reason.

Better yet, talk to Doug "Tanstaafl" Burnside on this BBS. He (or perhaps his installer) has created a custom subwoofer box for a much larger and more powerful woofer that fits into his spare tire well. He was able to pull this off because (if I recall correctly) he removed his factory-original full-size spare and got one of those tiny "space-saver" spare tires to put in its place. The leftover space is now used for the custom subwoofer enclosure. I'm pretty sure he runs a 10" in that space now. Doug, elaborate upon this for our friend Ezekiel?

On the other hand, it'd be much simpler just to give up some trunk space for a Q-logic box. That's what I've done and I haven't looked back.

___________
Tony Fabris