extreme stereo instalation help?
 

i have a 2001 S4 AVANT with the bose symphony system in it, i am going to swap out everything (2 5.25 speakers with tweeters in front, and 2 6" speakers in the back seat) and that is not a problem, the head unit i cant decide what to use, but no biggy
my question is
the car has a 8" sub behind the interior of the cargo space, what i want to do is keep that speaker in the car where i is and attach it to the new system, and then add a 12" sub somewhere else in the car
would a 12" sub fit on the other side of the car in same place or is there something behind there that i should know about before i tear into it?
if there is nothing there than i am ok with fabricating my own box to get the sub to fit there
i just don't want to compromise any of the space back there with a bulky box sitting randomly somewhere
i also will have 2 amps that will run everything and need a spot for them, dont think the area where the current amp is nearly big enough, i really dont want them on the back of the seats or in a box, but will i be able to fit them in the side some where like the sub is? i know its hard to say and i might just have to try to find out but any help would be awesome before i get into doing this
thanks every1