I don't know what Puglsey means by "lots of space", I think most 12" subs would work fine in a 1 cuft box....some are even designed to use less than that.
What space ARE you willing to give up? If you were creative and wanted to protect it from rain, you could remove the glovebox, and mount it facing down (this may only work on early K5's like mine, and it would still be TIGHT) or you could build a short but WIDE enclosure that would attach underneath the rear seat.
Also if the most important criteria is that it's hidden, or in a weird spot you might consider a couple of smaller subs (like two 8 inchers) which will give you more mounting flexibility.
Finally, I know that you said you don't want a bandpass box, but what about a ported one? If you do go with a smaller speaker, you can gain a pretty decent amount of bass by using a ported box. I agree that bandpass boxes are pretty useless.....as the name implies, the box is only capable of passing a very limited number of frequecies, so it tends to sound "One-Note-ish and Boomy)
Give us some more criteria of what you are willing to do, and what is off-limits and I'm sure we can help you out.
-Greg72