That's an option to put some mids in the rear deck in place of a sub. When I auditioned the Diamond hex series they had an option on the crossover to run an extra pair of mids without tweets for rear fill. I believe the pitch was you could run the rear mids off the same 2 chanels as the fronts, thus eliminating the need for a 4ch amp. It did seem like a slick idea, never heard that setup though. I can tell you the hex's were my second choice after the focals, similar sound. Of course the only bass gained with that option would be from having the trunk as an enclosure.
They do make subs in the 6" range that might fit back there,
this is just an example. I've never heard that sub but ED is not garbage. Whether a speaker fits is subjective, the fittment charts will tell you if a speaker will just screw in, but with a spacer or little plywood baffle, what you want might suddenly fit.
Get in there with a ruler and do some measuring.
here is a good site to gather info on different equipment.
You could just throw away your spare tire and jack to make room for a sub.
Subthump makes such a box for f-bodies and there are guys driving around with a sub instead of the ability to change a tire. Really though, what you're considering with the Quarts makes plenty of sense.
