Wait what exactly do you plan to use it for? I thought you needed max SQ for the money since you said,
If you have speakers behind you you'll screw up the soundstage out front since music is typically recorded that way. Sure there are SACD, DVD-A and concert DVD/BR in 5.0 but I haven't encountered a pure audio set-up in surround; such discs are usually listened to through the HT system, or they use the same universal disc player as source for both audio and HT, but of course concerts need to be seen as well as heard.
If anything should go in the trunk it's the sub, but that's mostly because not everyone will be willing to gut the interior to fit one inside (like Polk Audio showcars where they mount it on the dashboard, which Bose does on some stock Nissans now).
In any case, if it is max SQ for the money, then instead of, for example, $600 on three sets of $200 speakers, just get one really good set for the front. Or 2/3 on the front speakers then 1/3 on the subwoofer.
BTW you could actually mount both midwoofer and tweeter on the kickpanel - in the previous pic I posted it was just the tweeter. Here's an example of both, whch given it's a much larger car than what we have new nowadays, shouldn't be too difficult. Just aim them properly.