Don't know about the convertibles, but I'm sure all the e36's are the same up front. As for those, replace with a good set of 5.25" seperataes. Maybe the Infinity Reference line, or the PolkAudio db series. This is a two way system that will replace your tweet (in the door) and the mid (in the kickpanel). As for the mid in the door, I would just disconnect and leave alone. You would need a large amp to really hear the difference between a two way and three way aftermarket seperates system, and besides, they are FAR more expensive. Not really cost effective.
As for the rears, if they are the same as the hardtops, it's a 4" seperate system.