How much xmax does your main driver have? 10mm of xmax is a pretty dinky passive radiator, it's like making a ported box with a very small port, it just doesn't work. In a car where people usually want alot of bass, you'll break that passive really quick. To put it into perspective my passives, of which I have 4, have 35mm of excursion and those are 15's lol. That's why I brought it up in my last post. The passives need 2x the volume displacement of the active driver, so if your single 10 has more than 10mm of xmax odds are it'll end up pushing 2 10's passives with only 10mm of excurision too far.
What driver are you going to be using?