I don't see much use for an 18 in a trunk personally.
I would consider a pair of 10s/heavy 600rms+,or a pair of 12s,or even 1-12 in like a 2.0cu ft ported enclosure
Whats your budget of everything?Max?
GCON-10 Sound Solutions Audio 10" 750W GCON Series Subwoofer - SSA STORE
I would consider a pair of tens for that trunk space even..just my 0.02cents