when u invert the su bs ur supposed to reverse the polarity of the sub OR if ur amp has a phase shift option use that.
as for the rattle, if the subs are facing and fireing towards the trunk then ur gonna get trunk rattle. to get more out of ur woofer and reduce trunk rattle u have to aim ur speakers forward, but the only trick is when you do that you have to create a baffle.
your best bet is to look up baffle and/or aiming subs forwards or something along the lines of haveing them fire towards the cabin to gather more details.
to sum that up what you need to do, if you have the sub fire towards the cabin you have to seal off the box to the trunk so that no air is able to move around the side/top/bottom of the box. essentially it's like makeing the front side of the box into a wall with a speaker fireing towards the cabin. you HAVE to do this in order to combat a cancelation of bass. if air can leak and move around to behind the box cancelation will occur. best cheap way to do this is build a box that will snuggly fit around thewalls of your trunk and use "great stuff" (sealent foam that hardens, like 4 bucks a big can at home depot or Lowes) to seal off the side......it all ride on the seal and the "wall" you create with the front of the box meeting the the rest of the car relativly perpendicular.
this is a creating a "baffle", also research about that on the forums (search botton IS VERY helpful.)
apparently you get better output fromt he woofer when you do this b ecause it save you the trouble of haveign the bass bounce off the trunk BACK into the cabin, this allows u to fire directly into the cabin.
correct me if i'm wrong guys.....