OP, you didn't really state what your goals are. Are you looking for SQ or SPL? What is your budget? edit: $600, I see.......
In a trunk car, if you can fold the rear seat down - subs forward, port forward. If you don't want to/can't fold the seat down - subs back port back or up.
Ultimately, Everyone has to experiment for the best possible results.
The only way an acoustical engineer can get close to a 'perfect' system is if they had every possible object, dimension, and surface mapped out in the listening environment + enclosure environment. Given the time and knowledge required to do it accurately, trial and error wins every time in my book. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif