The Orphan 8's, some Audiopulse/TC Sounds or the regular Ascendants (if you can find them), e8's should hang, old ED e8A's (flat cone), and possibly the Treo SSi- though I know of no one who has actually heard one.
Precisely my hopes, but behind the seat of my Sport Trac. Hell, I can fit a second pair if I got real squirrely!
Anyone know how bad it would affect things to have 3 drivers behind the seats, and one downfiring under the seat?