The only time you have issues with localizing a sub would be due to vibrations or if the sub is playing what most consider to be mid-bass. The reason being is that wave lengths become short enough that your mind can figure out where it's coming from. But if a wavelength is 32 feet long...it doesn't matter where the sub is in the vehicle. My sub is LP'ed at 80hz using 12db slope and my bass is all up front, as if the mids are creating it. If there is some low notes in a song vibrations and pressure let me know that it's from my sub in the trunk; but only if I keep my rear seats down. Up, I don't have that problem at all.
So if you wanted to put a sub in your kick panel, floor board, whatever...yes you can get away with it. Something a lot of people do is put a tweet and mid-range on their dash, and use 6.5" to 8" subs in their doors. Now you can have stereo subs, not that it makes a big difference...unless those subs are crossed fairly high...i.e. 200hz or higher. At that point you're really getting into speaker selection dynamics.
