Okay, on the one 18.
But i have one last thought for you...
My next box is going to be 31 wide by 16.5 tall by 28 deep with two 6 inch aero ports, tuning would be around 33 hz. the subs will be on the 31 inch side pointed forward and side ported.
That would work in your trunk as well and it will still leave you with some room in your trunk.
Even if you go with one 18, I still recommend pointing the sub forward.
Either way, best of luck to a fellow trunk guy.