bimrboy - anytime.
MP328, it depends on which package you have, and what year your car is. But early model with Premium Package (10 speaker sound), which most of us have is:
front kick panel - 5.25"
front door tweeters - 1"
door mid - 3.5"
rear speakers - 4"
rear tweeters - 1"
you can fit a 5.25" in the rear deck with a custom box, or an aftermarket bracket. It's also possible to fit a 6x9 back there.
You are correct, you want a 1 way, since you already have a tweeter back there.. or you can get a 2-way and drill out the tweeter hole bigger to use as air pass through if your rear seats don't fold down. Will depend on which box you have... early model e36's came with a sealed box, later model e36's came with a baffle box.