I would say the 718 would perform better in an enclosure but it should be mounted on a board that perfectly fits in the trunk. I think you would be installing them in a high mounted board above CNG cylinder and that would make it sound terrible.
As madhouse said, 718 dont have response of equalized music. The bass is great as always desirable but the treble is shrill. You can however try to shift the Treble Centre Frequency to a lower level using the (claimed to be) parametric Equalizer in your HU. That way you would prevent your ears from bleeding due to high pitched notes.
So if you are going for 718, make sure you atleast have a sealed box (not that ported one with port in center and certainly not that flat board with gaps around seat to board boundry). Damp it with Jumbolon or polyfill. You'll enjoy the bass alot.
Or as an alternative, you can go for Pioneer 6902r. Their tweets are a bit soft.
