OK LET ME EXPLAIN THIS VERY SIMPLE LIKE
6x9's over powering your front speakers is bad for imaging (sound stage). I hate 6x9's personally.
If your rear seats have a fold down arm rest in the middle then you can port it right there. Maybe some e30's do not have the fold down arm rest but mine does, and I know others do as well. When you fold down the arm rest it normally does not go through into the trunk. But it is very easy to fix that.
I have probably been typing for longer than you buddy, I don't see how my post is hard to understand.
Go to this page please http://www.inspar8r.bmwe30.net/index.html
"My modifications include: Polk Audio 5-1/4" component speakers with 1" tweeters in the upper corners of the windshield, Kenwood KDC-x917 eXelon head unit with motorized flip face, MTX 250W rms 4-Channel amp, an MTX digital Class D 500W rms 2-Channel amp. 2 12" Kicker CompVR subs in a sealed box, and i removed the rear deck speaker to improve soundstage and they act as ports to allow the subs to radiate into the interior through the rear deck. I also punched out the ski hole perforation with a rumber mallet."
Read the last line I ALSO PUNCHED OUT THE SKI HOLE PERFORATION WITH A RUBBER MALLET
I know what I am talking about if your car does not have it, sucks to be you