In the limited space of a vehicle the direction of the subs and where they're facing really is insignificant. The subs are all about how much air they can move, lol, at those low frequency's it's not so much directional at that close distance as they (frequency/sound) travel just fine. . They do make subs that even fit under seats now, havent heard them but they do work. I have a sub in my trunk on my older GP and it couldn't sound better (faces rear). Facing it forward would not change the sound IMHO.
You can experiment if you like and I'm sure you'll find no difference in sound when inside the vehicle.
