Man there' wayyyy too much to go into about this, but I'm gonna keep it simple as you should. Buy you some high end subs and they'll be so loud you won't care which way there facing, you'll be trying to underpower them.
If that's not an option the best thing to do is do tests with the box. Listen to the speakers faceing fwd and backward.
I'll say this, it is better in certain cars for woofers to be pointed into the cabin. Ideally if you were in say a hatchback that'd be the best thing to do.
Since most of us don't have hatchbacks we have to adapt to the dyanamics of trunks. To get the best possible pump out of a sub with the trunk lid closed you could place the speaker facing away from the cabin and right up against the front of the trunk(back of the car). There's debate there of course but since the woofer is closer to the edge of the trunk it's not canceling out the wave that it's creating.
That's about all I feel like delving into about this right now, but if you have more questions post up.