I had some extra time this afternoon, and decided to try adding the Rockford HE sub that I had in my Miata with the subs I already have in my Celi...
Folded down the backseats to give it more air space, ran the wires for the HE's bandpass box up front and set it up on the passenger seat...
Drove around setting off car alarms for about a half hour before I started to get a headache.
Conclusion: I need to find a less half-ass way to add a third sub somewhere in my car.