Midbass is directional, mounting them/it between the seats will likely cause as many problems as mounting them behind you. The problem is, your ears can discern the direction midbass freqs are coming from. As someone above said, having your midbass come from behind you completely destroy the illusion of a front stage. Well, having a chunk of your front freqs coming from the center of the car, rather than the outsides, will also screw this up. Mono midbass, or midbass coming from an inapropriate place is just a bad idea.... unless SQ doesn't really concern you, then yeah, just put em where ever they'll fit.
Now, if you are adtually wanting subwoofers, not midbass drivers here, nevermind. But it sounds like you are wanting midbass drivers, not subs, in which case my points still stand.
Ive heard the ID series does not make a great midbass speaker, certainly nothing like the old IDQ8's that are no longer made.